Do not get a 8800GT with that CPU, you'll bottleneck the card, get something like a XFX 7900GS or 8600GTS and 600W is unecessary, i recomend a 420W PSU something like an Antec or OCZ.
Also download CPUZ and click mainboard and you can see if you have a PCI-E slot and what bus speed it is, if its AGP which is an older gfx card interface then obviosly you'll be needing an AGP card