depends on the game. take gta4 for example. 1gb will give you more eyecandy (but not performance). but at that low resolution why not a 4850 1gb instead?
The 4850 1GB is much more then the 4870 512MB. The 4870 has a higher core clock and the added memory of the 4850 wont make a difference at that res...go for the 4870 512MB