To tell you the truth, the 4850 is a better card than the 8800gts 640mb, no doubt about it, it's a big step ahead. BUT, the 8800gts 640mb is still a great mid-range card. So the question here is, what is the resolution you play
? IMO, save your money and wait for the next generation of cards and THEN change. I own 2x8800gt in SLI and i play everything Maxed out with some AA @ 1680x1050. So that is exactly what i am going to do, wait for the next gen. Hope that helped.
that 4850 will outperform the 8800GTS 640 (g80) by a decent amount, but is it worth $200? Hard to say. If you are becoming unsatisfied with the performance of your current video card, then perhaps, yes, it might be worth the $$ to upgrade. If you're still getting reasonable performance from your current card, then wait until the next gen of cards to put any money down.
Generally, you want to jump at least 2 tiers when upgrading a video card, or you are really paying a premium for the performance boost you get. Going from a 8800 (g80) to a 4850 is not really 2 tiers, it is more like a tier and a half, so it's kind of up to you.
As mentioned, rest of your sys specs would be helpful.
If you have an SLi board, and an appropriate PSU, you could always SLi your 8800GTS. While the 640 cards are getting harder to find new there is always ebay or craigs list if you're willing to take it to chance to get it cheap.