Depends on what you need / expect. If you can live without a bootscreen and don't need Apple's DVD player or steam games (both of the latter can be solved by installing an additional .kext), you can e.g. put a standard PC 6850/6870 in there (provided you purchase the required power cable(s) as well). Required: 10.6.8
You can also go for the original Apple 5770, which is more expensive than the cards mentioned above, but shows a boot screen and is running in the 1,1 despite Apple claiming it's only for later revisions.
Other options (XT1900, 8800GT and what else has been available in the MP officially from Apple) may not be that interesting for one reason or the other, but could be an option as well - sometimes flashing a graphic card or installing additional software may be required, though.