Now that's a thought. If you can find two GTS250s or 4850s (1gb version) for around $100 and your motherboard supports CF or SLI, your PSU is up to snuff and you don't mind input lag or microstutter you could get roughly GTX275/280 performance for $200.
This is a great idea. I got a used 4870 512MB for $85 shipped a few months back on the fs/ft forum here, so you miiiight be able to snag a pair of those for under $200. I don't think that's an easy deal to find, though.
But yeah, basically you're looking at somthing in the ballpark of a 4850 and 4890, performance-wise, for this price bracket.
Here's Tom's Hardware's GPU hierarchy chart (updated every month):
http://www.tomshardware.com/reviews/graphics-card-geforce-radeon,2646-7.html
They say that you'll want to upgrade to a card that's at least 3 tiers higher than your reference to get significant gains. So, with that in mind, you can see that the SLI/CF of 2 4850/gts 250 is going to give you the best performance in your price range. Also, you can see that, with the exception of the SLI/CF situation, you won't be able to get a setup that's very significantly faster (3 tiers higher) than your old 8800 GTX, but that any of them will still be faster. The question is, even though the 5770 is faster, cooler, has more features, etc than the 4850, you have to ask yourself if it's worth nearly double the cost.