1 GB will help but only under very specific situations, and the issue is not the bandwidth it's because at that level were talking about the HD4850 1GB averaging in the low teens and the 512MB low single digits, sure it's 500+% better, but it's still not very playable. The HD4870 just barely makes it more useable, the exception also applying ot anyone considering Xfiring a pair.
If they're relatively close in price I'd spend an extra $20 or so for 1GB, but I wouldn't spend $50+ unless it's a very long term build, when the feature set might push into the VRAM more once games start to exploit the DX10 tool set.