4850 1gb vs 4870 512mb

I'm currently debating on graphics cards.
I like the IQ4+ by HIS series because of they're low temperature settings.
I'm not sure if I should buy the 4850 1gb version for 210 before rebate or 4870 512mb version for 259...
I'm using a 20" widescreen lcd monitor... at 1680x1050...
I like to play fps like CoD4... benchmarks show that the 4870 is ahead.. but at my resolution should i go for the 1gb?
also i was considering the 1gb will help for future with newer games.. but then again the 4870 will be faster...
ty