1G of DDR2 RAM (all the MOBO will take)
1.8G P4 processor
GeForce 7600GT 256MB (upgraded from a 128MB 6600GT)
80G HD that is about 40% full
235W power supply (my last video card and this card recommend 300W, but my Mobo has a proprietary Compaq plug and and this is as powerful as I can get)
You need a new motherboard, which will necessitate purchasing a new processor and possibly new RAM and a new case. That would also allow you to upgrade the PSU.
Your only other option is to play older games until you can afford to upgrade. Look for "Top 50 Games of All Time" lists and search the bargain bins.