The fastest of the options you presented would be 5770 in CrossFire, which would be on par (for the most part) with a 5870. The 5850 would be your next best option, and might even be a few bucks cheaper than 5770 CrossFire, but a bit slower too. After having CrossFire though with 4830's, I must admit it's nice to be back to a single card setup, just makes life easier. The 5850 will overclock to the speeds of the 5870 (and therefore 5770 CrossFire speeds) easily. But overclocked 5770's in CrossFire would of course be faster than a 5850 overclocked.
Given that you already have an 8800GTS (G92), it might be easier and cheaper to just pick up another one. That should be fast enough to play anything out there right now, but the other two options would be faster. And given that the 8800GTS is already getting old, you can count on a continued loss in value of the cards over the next year or so. So if you later change your mind and want newer generation cards, your resale value might not be so great.
Honestly I think any of the options would be a solid choice, just depends how much you want to spend and how much hassle you want (in regards to selling the 8800GTS, overclocking in CrossFire, etc).
EDIT: Your PSU can easily handle a single 5850. It could even handle 5770 in CrossFire, or 8800GTS (G92) in SLI. I'm running an overclocked 5850 with 5 hard drives, 5 case fans, and an overclocked Q6600 on a 550W Corsair 550VX.
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