If you're looking to buy a card as a stop gap measure, then the choice is simple. Buy as much card as your budget allows. After all, you will continue using this card when you upgrade the rest of your system.
^ Concur get the best you can, HOWEVER a cavet:
Most "good" GPU's are pci-e 2.1 cards and you have pci-e 1.0. NOT all pci-e 2.1 cards play well with pci-e 1.0 (don't sweat Bootlenect by CPU as that goes away when upgrading system).
This is a MB issuie (not a slot problem) and may be fixed in the lates Bios revision.
Will give you a little insight into what I'm talking about: http://www.tomshardware.com/forum/296413-33-will-mobo-s...
Do a search and make SURE the selected card will work with pci-e 1.0 ( even though in England you can go to newegg.com and read the reviews - look for responses indicating use in pci-e 1.0)
ie on gigabyte 6950 (did not see any users with pci-e 1.0), but is a LONG card: http://www.newegg.com/Product/Product.aspx?Item=N82E168...
as dirty asked, what PSU. You need a quality 450 watt PSU and check the numper of pci-e plugs from PSU. ie the 6950 requires 2 6 pin connectors. Also case, some of the newer cards are LONGE, and cover two slots (on back).
YES, enough horse power
•Good DC voltage load regulation
•Good efficiency at 110VAC, great at 230VAC
•Three year warranty
•Failure to verify 80 Plus compliance using the OCC Testing Methodology
•Borderline DC quality
•Basic user manual