Refer to my signature line for my current set-up.
This summer I am planning on some minor upgrades.

I plan on swapping the 2800 chip up to the 3200.  No flash bios update will be needed.
Trading my 6600 GT up to a 6800 Ultra.  The 6800 Ultra is only $75 more.
Adding a 120 gig 7200 rpm Seagate and configuring it as my main hard drive with a 20 megabyte partition just for FS.  I will use the current hard drive just for storage and my wife's personal applications. 
Hopefully 512 3200 in each of the 3 ram slots. 1.5 gig all the way. 
Will increase airflow also though I'm not sure what I need yet, I don't know the flow specs on my current fans.  What do you all think?  Will I get a noticeable improvement from my mid cycle upgrade?

All decent bar your partition size.

You dont want to fill that partition more than 40-50% to keep it working at its fastest, hence the biggest you could let your FS9 folder get is 10gig, which is nothing.

How big is your current HDD? I would say best bet is set 60gig for your OS and progs on new drive.  The other 60gig on the same drive use for your junk and stuff.

On your old 60gig HDD install FS9.  I cant remember but I am sure I read or was told that FS9 will work better if its on a totally seperate drive to the OS.  Anyway this would allow you to have an FS9 dir of 30gig before loosing to much performance which should take you a while to fill.

On the ram side of things I wouldn't bother with 1.5gig.  Just set your two 512 sticks up so they are in dual band mode and that is more than enough to get the best outta FS9 (my system rarely uses above 650meg when fs9 is going)

You should get a noticeable difference as you are going to a higher FSB speed and a better graphics card, though I would say the g/card may be overkill.  Bear in mind that even with an A64 3500 and a 6800U my CPU is the bottleneck! saying that I suppose you are future proofing yourself a bit.

You would need a new mother board to use PCI Express.  I am pretty sure the one you have now will not support PCI Express.

If you want PCI Express then go for a Socket 939 mobo and get an AMD64 cpu, the best you can afford that fits a Socket 939!