First be sure your board will support that CPU (I'm sure you checked that    )

Will that 512 of 2100 be in "addition" to your existing 512 (both 2100) for a total of 1 GB,, or does your board support both SDR and DDR and you'll just be switching from 512 of one to 512 of the other ?

Then you should see a noticeable increase,, the diff between 1.4 and 2.0 is substantial (especially if you have a decent V-card)..However,  once you reach 512 of RAM, you don't see a big increase in FS2002 when you go further, but more RAM (especially at that price) is always a good thing 

Most upgrades touch of a chain-reaction of other "needed" upgrades that end up making a MAJOR upgrade more economical,, but in your case, I think it would be money well spent.... if you ever go with an even faster board/CPU combo,, that RAM will be fine (as opposed to buying 512 of PC133  and having to scrap it later    )

Hi Andrew, I don't know what kind of video card you have, but if it's scoring under 5000 in 3dmark 2001, your money is probably better spent on a gf4 ti4200.

You won't notice the extra ram unless your doing video editing or some other ram intensive application.

Also, chances are you will want to upgrade your m/board to something with a 400mhz or faster front side bus speed in the future and your ram won't do any good. Save the ram money and put it towards a new 400mhz mobo (and PC3200 ram) or video card IMHO.

The processor sounds good, is the XP2400 the new Thoroughbred chip? I know some of the new 2100's are.
I think they run cooler on a new die at lower voltage and have more cache ram.

The CPU upgrade is important I feel. It seems like once up to or around 2ghz, the system really gets smooth as the processor is now finally able to keep up with the load on it.

Does your mobo's BIOS support the XP2400? you may need a BIOS upgrade.

Nice card those GF3 TI 500's

I spoke poo above about the thoroughbred AMD's having more cache ram, because they don't.

Also I found out that although theoretically the chip should run cooler, they actually don't. It's because the core is actually smaller and has 40% less surface area to dissipate the heat, so a copper contact surface on the heatsink is apparently needed. Sorry if I caused any confusion.

AMD no longer warrant against overheating.

Man,, the "heat" issue is one of those "spare no expense" problems.

I've gotten so preoccupied with cooling, that I won't even put the cover back on my case, and even keep a floor-fan focused on the board/CPU. (prob over-kill, but that's me)..

Obviously, the biggest fan that your budget, power-supply and case case will allow. And just as important,,, at least one chassis fan. Blowing air on the CPU is good, but even better if it's not the air already heated by the other goodies in there. I got an amazing cooling increase in a friend's computer, by adding one of those neat "PCI slot fans",,  they suck an incredible amount of air out of that environment.

*note: when you start adding fans, you may be pushing your power-supply. DC-motors are current hogs.