From what I've read the 1Gb version is no better then the 512mb. However there's a good performance increase if you jump to a 4890.
I don't want to keep these cards because I know I won't use them. They'll just sit and collect dust. I'm going to a 58XX series card, and the other 4870 I have replaced the 3870 in the second rig here.
I have an old Radeon 9250 PCI card

I can use as a backup since THAT card isn't worth the hassle to try and sell. Although it can't play Crysis, it'll still get me out of a pinch!
Anyhow I have the cards posted for sale now. If someone doesn't agree with my price then....... it's a free country. They can make me an offer.