Good video card on a sub-par system

So here's my situation -- I am away from home for the summer and did not want to spend the big bucks to ship my computer safely to my current location. I am staying at a cousin's house with an older "enthusiast's" system - P4 w/ HT @ 3.8 GHz (stock speed), 4 GB PC5300 RAM, and a PCIe video card slot. It has one of the original PCIe cards that came out -- it won't run many of the current games. In addition, I have been wanting to upgrade my video card for my system that I left at home (a computer that could handle any single card). If I were to buy a good video card to add to my system at home later, how do you all think it would run on the above described P4 system in the mean time? Also, I know I'd need at least 650 W of power, but what should I be looking for on the power supply in terms of connections for some of the better cards on the market today? Thanks for your input!

computer not with me has:

Core 2 3.0 GHz (E6600)
4 GB PC6400
Abit AB9 Pro Mobo
8800 GTX OC (640 MB)

Keeping an open mind to building a new computer after this summer if I can afford it, something that'd better suit a top of the line card than current system.