I'd have to say 1280x1024 as a realistic resolution, with all AA and AF settings on max, but my monitor only supports 1024x768 so i get good framerates in almost everything.
The best resolution is whatever is native to your monitor. My monitor is 24" so 1920x1200 is the best for me. Of course, I often have to turn settings down a bit but I would rather have higher res than prettier graphics. Some people don't like 24" monitors because they are too large but it doesn't bother me. Just go with whatever you are comfortable with.
My x'fired 4870's handle the high res pretty well, I'm able to run pretty much anything at full res with AA and texture settings on max. Its whatever your eyes are most comfortable with though.