well you need a new motherboard, because that one is agp not pcie.
If you get a new motherboard, you should then get proc, ram, and a grpahics card.
I dont know if you want to spend this much or not, but heres a sample build
GIGABYTE GA-P35-DS3L LGA 775 Intel P35 ATX Intel Motherboard - Retail 100$
some cheap ddr2 800 ram 25$
Intel Pentium E2180 Allendale 2.0GHz LGA 775 65W Dual-Core Processor Model BX80557E2180 - Retail 90$
now here you can ether spend a good amount on a good graphics card, or a less one. I would recommend the 8800gt, about 260$ but you can obviously choose to get a less one like a 7600gs or something.
Also, how many amps on the 12v rail on your psu?