Hello to everyone. I am about to upgrade my own PC (last week I asked for advice regarding a friend's graphics card - I ended up getting him another stick of RAM instead   ). So ofcourse I came once again ask for the wise advice of fellow simmers!  
Here is the situation:
Athlon XP 1800+ (1.53Ghz) OCed to 1.6Ghz
ASUS A7S333 MB
512MB DDR RAM (1x256 PC2100 and 1x256 PC2700)
ATI Radeon 9600XT 256MB 8X AGP
40GB Maxtor HD
8x10x32 ACER CD-RW
16x8x16; ?x32x52 NEC CD-RW/DVD-RW
300W Power Supply

Nothing to brag about   One of the things is that I bought the graphics card a year ago since I thought that I was going to upgrade then, but did not have time for that last year, so I have a 8X graphics card wasting away in a 4X Motherboard.

Now, this is what I want:
AMD 64 3000+ S399 ($140)
[S939 MB] ($90-120)
2x256 PC3200 RAM -or- 2x512 PC3200 RAM (taking advantage of dual channel) ($23+23 or 43+43)
PCI-Ex Video Card
Everything else I want to keep the same. (In due time I will add a 160gb HD.)

So now my other dilema: My current VCard is an AGP. Alot of the S939 MB's use PCI-Express. So, at the moment I have a friend (the one from http://www.simviation.com/cgi-bin/yabb/YaBB.cgi?board=hardw;action=display;num=1... - he decided not to get the video card since I realised that his MB is a 4X and not an 8X, but now I told him that I may go up to PCI-Ex and he got all happy and said he would take mine off my hands for 2200RUR ($76...I bought it a year ago for $140) So, I have a chance to get rid of this video card and get a PCI-Ex, but, I do not want to spend any more than $130 on it   (budget).
Sorry for the long rant, now to the questions:
a) Is it worth it for me to switch to PCI-Ex of that price range? Will the performance be noticable? If Yes: What card do you recommend in that price range?
b) What MB do you recommend for me within that price range?
c) Does the Dual Channel RAM work with all 939MB's and all types of RAM?
d) What if I leave the RAM I have now for a bit?
e) (To someone who has personal experience upgrading from 512mb to 1024mb) Is it worth it? Is this smoothness or just faster loading?
f) Will my 300W power Supply handle this?
Thank you very, very much for any help that you can provide to a very confused guy that is having nightmares deciding what to buy!!!
Vic
Edit: Added power supply info.

300 Watt is barely enough, go for 400 as a minimum, especially if your running a new ish PCI-e card. 

Thanks for the advice Dan, Scania and autopilot!
I have decided to get 1gig of RAM (2x512 DDR400 - haven't decided on brand yet), will keep the video card I have right now (Not that old and you said that the jump from 4x to 8x will be noticeable and that PCI-Ex isn't the best for me yet and it does add atleast $130 to the total)
Now, which of the ASUS MB's would you suggest?
(I would like one with an NForce chipset...but it seems that all NForces are PCI-Ex...I had a VIA before...didn't have any real complaints - any comments about VIA??)
Here are the finalists:
ASUS (The only ASUS boards that are AGP)
Asus A8V ($91) http://www.asus.com/products4.aspx?l1=3&l2=15&l3=0&model=478&modelmenu=1
Asus A8V Deluxe ($107) http://www.asus.com/products4.aspx?l1=3&l2=15&l3=0&model=238&modelmenu=1
Asus A8V Deluxe + WiFi ($117) (Same as above but +the WiFi kit)

GIGABYTE
Gigabyte K8NS Ultra - 939 ($112) http://www.giga-byte.com/MotherBoard/Products/Products_Spec_GA-K8NS%20Ultra-939....

I wouldn't mind spending a bit more on the MB, but I cannot really find a 939MB that has AGP  ???

2 Last questions:
A) Which motherboard would you suggest out of those? (Or a new candidate)
B) I heard that the AMD64 S939 uses LESS power than what I have now (Athlon XP 1800+) is this true? (I can, but really really really don't want to change my power supply right now)

Vic
Thanks!!!

The problem you face is that you have the opportunity for a great upgrade and your gonna screw it up by sticking to your AGP card.

Better to just wait until you can afford the PCIe card, and by then the whole upgrade will  probably be considerably cheaper anyway!

If you spot any high spec RAM on special (like PC4000 - 500mhz DDR) you'll be way in front for later on down the track, when you can get a free PC upgrade thru an overclock. Also, stretching the CPU to a 3200+ model is desirable as it's CPU multiplier is 10x ..... which equates to an easy and powerful overclocker with PC4000 Ram.

Just about all of the new CPU's are multiplier locked, which means that any overclock must be done on the FSB, hence the need (and demand! ) for overspec ram modules, as the RAM usually gets clocked up on the FSB as well.

Without going into overclocking in depth, (another forum for that thankfully), I'll just say that it's quite safe these days and  it's often manufacturer supported by them - giving you the software tools and settings needed for easy and safe success.

AMD cpu's (within  an identical  core specification) are all essentially the same CPU, regardless of their speed rating. That's why the multiplier is locked, otherwise every man and his dog would just whack up the multiplier and instantly get high end CPU's for a fraction of the cost. This wasn't always the case, and in the past, CPU's were "binned and marked" according to how fast they would run and remain stable. Binning may still occur to an extent, but AMD have had a lot of practice at making these things, and they have it down pat, you'll find most AMD CPU's are capable of very high speeds despite there "rated" XP number and locked multiplier.

These things will cost you a bit extra at first, but they will reap huge benefits in the long term. With the above high spec gear, you can get very close to cutting edge technology.

Overclocking is not for everyone of course, and I understand your budget restraints, but you still need to get the best CPU you can manage to buy, as this will be the bottleneck in your new PC.

If you buy only PC3200 RAM, then get 2 x 512mb of the lowest latency, (CAS 2.5 @ 400mhz ? ), you can afford. This may take some skillful shopping.

CAS latency figures in a RAM spec generally give you a quality/performance indication, rather than relying on a manufacturer's reputation, as most manufacturers make cheapo ram as well as good stuff.

My second "Socket A"  PC had the same MOBO as your's. It has an SIS chipset which is a real dog performance wise. I ran identical hardware as my friends PC, except he used a decent chipset (mainboard) and his PC was noticeably faster in all areas. That's why I started researching chipsets and why I rave on about them so much. They do make a huge difference in many ways.

Just as the "nForce2 Ultra 400" Socket A mainboards were the king of the heap in 32bit computing, Socket 939 nForce4 SLI or Ultra chipset based mainboards are currently the best AMD 64 platform for simmers/gamers........ hands down. There is a lot of hype over buying an SLI mainboard, but seriously, unless you have spare cash to blow on a bit of extra performance, SLI is so totally not needed and the nForce4 Ultra (single PCIe Graphics card) is just fine and dandy considering how awesome the current crop of video cards are.

Mainboard brand is less important than the chipset by far, but you can actually see visually the difference between a quality board and a lesser product. You can even tell by clear pics of them on the web. ASUS, DFI, Gigabyte are good examples but there are other major players like Soltek, MSI and others producing very nice boards.

It's interesting to see benchmark results of all the boards using the same chipset...... there is hardly any difference in performance between them. The nForce4 chipset has so many features built in and most manufacturers are implementing just about all of them in their high end boards, which can be bought at bargain prices like never before.