The 5830 is getting outdated at this point, although there are some good deals on them. I'd recommend the 6850 over it, though, as it offers pretty similar performance only with newer technology. The HD 5770 and 6770 are the same card only rebranded; I myself have an HD 5770, and while it does acceptably well in most games, it just isn't able to max out modern games at 1080p. Both cards are a great value, but are quite a bit inferior to the 6850. The GTS 250 is essentially an outdated low-mid range card; avoid it.
This might help some:
http://benchmarkreviews.com/index.php?option=com_content&task=view&id=627&Itemid=72&limit=1&limitstart=10
The GTS 250 doesn't even make it onto that list, and for good reason.
Edit:
While I understand that there are newer cards coming out, you can only wait so long. Once they come out, there will be new ones after that, and then more; aside from that, any performance claims on these new cards are nothing but speculation as there aren't any legitimate benchmarks out on them anyway.
If you want to wait until then (I don't have a date... sorry...) to see if something exceptional comes out, then go ahead. If you want a card now, though, then go with the 6850.
http://www.tomshardware.com/news/AMD-Radeon-HD-7000-gpu-video-card,14596.html
They claim to be releasing the 7750 and 7770 cards after Valentine's day; it's pretty close, so I'd wait and see how they do... in all honesty, though, I don't expect to see one of the new 77xx cards beating the 6850, and the 78xx cards are going to be way out of the OP's price range when they're released.