For my next upgrade I am considering a 4870, but I really don't know if buying a 1gb version would be worth it, since the difference between the two is $40. Keep in mind that I really need to trim money off this build!
Based on the $40 difference, and the need to trim money off your build costs, I'd go for 512MB version.
However, ultimately the benefit from 1GB will be determined by the resolution at which you'll be playing your games at. If you're playing on a 17" or 19" monitor I don't think there's a point to anything larger than 512MB, as those monitors typically won't use resolutions high enough to merit a 1GB card.
I have an HIS 4870/512 card. I replaced it with an XFX GTX260. Much better. How the 1gig card is going to perform....a few more fps perhaps and maybe you'll be able to turn on some of the goodies without taking a too much of a hit. ?.... what are you going to play ?
edit: I have to add..... unless your processor is going in the neighborhood of 3.4 or more neither card is going to give it's potential........
Certain newer games, and older games with mods eat up vram, but at 16x10, I think youre ok. As for the 260 over the 512, not worth it. The 216 vs the 1 gig , still not worth it. When both cards came out, the 512 was 6% better than the 260, recently nVidia has done some nice driver improvements in a few games, and now so has ATI. Generally its like this for performance. 4870 512 > or = 260, while the 4870 1gig = 260 216. Let price be your final verdict.