I wouldn't look into RAM, your computer appears to use RDRAM, which will be amazingly expensive...
Possibly changing your motherboard out for a DDR supporting Socket 478 board from Abit or Asus (there are some other good manufacturers, however those two are personal preferences (MSI would be another fine maker, and DFI's Infinity series would be fine))
The motherboard should cost you around $40-$50, that'll allow you to use DDR (and you can sell off the expensive RDRAM for probably enough $ to recoup what you spent on the motherboard) and then with your $120 you should be able to pick up 512MB or more of DDR 400 and a somewhat basic, but newer, AGP card
possibly a 9600Pro
that's my 2 cents, if you can get RDRAM cheap tho, i'd definately do that before replacing the system board (you'll have to reformat with the system board replacement, which is another downside to it, but i'm just suggesting it as a solution to move you into a more upgradable area)