I have a 8800GT @ 1680x1050 and I can run L4D at max settings without any lag/hiccup watever you want to call it. The gameplay is very very smooth. Your card should perform even better. Maybe there is a driver issue???
I agree with most of the people here and that's why I did not vote in the poll. I think you should go for a 4870/gtx260. It just makes more sense at 1680x1050. I can see the dilemma you are in but not many ppl here would recommend a 4890/gtx275 for your resolution.