there is really not a lot of difference. Depending on game one card may be a little better than the other. both can be overclocked quite well. I guess look at some benchmarks and think about what games you play most, and chose the gfx card that is better at those games. you honestly wont be disappointed with either, they are both good. they are both a bit overkill for a 20 inch monitor. You could probably step up to a 23" monitor for a few extra dollars, and save some dollars by going with the 1gb version of either card. either way make sure you get a good PSU to power this setup, not a cheap brand.
either well run just fine, they go back and forward in benchmarks. so you can't go wrong with either. i'm actually debating between the two at this point. always been a amd/ati guy but wouldn't mind going with a nvidia card.