The 4850 and 250gts are the performance point at which 1gb v 512mb of vram will start making a difference. For 1920x1080, the 1gb card will probably give you a bit more performance.
I gamed fine with a single 4870 512mb card for a long time, but when the AA/AF/textures were cranked up, I started falling behind the benchmarks of the 1gb cards quite noticeably. I can't recommend any 512mb card for that resolution.
A GTS 250 1GB usually runs neck and neck and in some games bests a 4850 1GB card. However, at your resolution a 512MB GTX250 may have much lower performance. I couldn't really find a good comparison for this in charts.
Honestly I am not sure which I would go with. Probably just keep your order and OC it a bit.
Any idea how much fps I would lose dropping from 1gb to 512mb in games like Dragon Age, Mass Effect, Oblivion, Just Cause 2, at that res?
The 250 is 25% cheaper, would it give 25% less performance at 1080p? I doubt it.
Also the whole thing might be a little bottlenecked by my 2.4ghz E2220 dual core anyway. With that cpu would either 512mb or 1gb make a diff?