8800GT is a safe bet. Some of the later cards are hit and miss. There are some which Apple will tell you won't work but they will. I don't know which is which any more. If you don't mind fiddling or hacking to some extent, then Netkas is your man.
netkas.org
Apple 7300s are failure prone due to bad caps. X1900s are noisy and hot. Also prone to going pop as a result. I think the current aftermarkets will work but I'd get confirmation from someone who has tried it. 5770 or 5870.