Yeah, around that. Although the human eye doesn't really notice the differnce.... a good framerate is arbout 70-80fps. I get 90fps in cs:s. But yeah... 50-60 is no lag...
That card will be enought good to play games like HL2 with best setting with quite good frame rate. I really can't say how good the fps will be l because I don't have that card, but I was able to play hl2 with the best settings with my FX5700 with no lag so u will get a good frame rate with the 6800.