i945P is quite old chipset, i think it have only PCI-e v1 when 5770 is 2.0 so it maybe little slower, but yes it should work as PCI-e 2.0 is backward compatible with 1.0 and your power supply specs seems to be adequate.
No problem, the HD 5770 will not saturate all 16 PCI-e 1.0 lanes.
Thanks a lot guys, I plan on building a new PC sometimes next year and when i do so, I will prob. xfire another HD 5770. I was mainly concerned about the processor, motherboard, and OS compatibility. Thanks guys!