Well, if you don't want to spend over $200, you might be out of luck with the new cards as, unless they are horribly underperforming, I'm guessing they should be closer to the $250-$300 mark (especially since nVidia won't have anything next gen yet.) History would place the new cards at close to 2X the performance of the 4890 but history isn't always right (and we currently have nothing other than speculation.)
I think you should consider the following:
1) Is your 8800GT not sufficient for what you currently play?
2) Is that $200 a hard limit?
If you either absolutely cannot spend above $200 or that 8800GT is too slow for you, I would catch a 4890 sale and just get it over with. Realistically, with prices so low on sales already, I doubt it'll be THAT much cheaper when 5xxx does come out. And unless 5xxx is a total letdown performance-wise, I can't see it being in your price range when it does release. If, however, you are satisfied with current performance, then there's no reason not to wait till 5xxx releases before you do anything.