Another 8800 GTS 512 is the best option, the 8800GTS will outperform the 4850 (this fact may be covered by a shroud of hype and fanboyism, but it'll become more clear as time goes by) and in SLI they'll be faster than a GTX 280, as dagger aptly said.
I wouldn't say 8800gts outperform 4850 overall. That is only the case if you compare resolutions of 1280x1024. The one key advantage of 4850/4870 is it scales well as resolutions increase. Look at the 2048x1536 charts, the 4850 leads 8800gts in almost every game. Of course, in games like Crysis, the 4850 gets 13.6fps at that resolution, which is unplayable anyway. But it's still valid for performance comparasion purposes.