Here's a decent article that shows benchmarks for the most popular MMORPGs with a selection of moderate cost GPUs. Most sites don't use MMORPGs in their benchmarks since so much is dependent on server latency and other factors.
http://www.xbitlabs.com/articles/video/display/mainstream-cards-mmorpg.html
http://www.xbitlabs.com/articles/video/display/mainstream-cards-mmorpg.html