If you're willing to spend $250 for a Radeon 5850, you might as well spend $30 more for a GTX 470MSI GTX 470 for $279.99 after rebate
The GTX 470 isn't so much better that is worth the extra money imo considering the power/heat/noise difference and that the actual initial cost difference is $50.
A few frames better, but it sucks so much more power and grills so good, you're better off getting a HD5870 if you want to buy a GTX470.
The HD5870 is in another price category altogether and is a quite poor choice in terms of price & performance IMO compared to either the HD5850/GTX 460/470.
How much better is the 470 than the 5850?
The GTX 470 at stock is usually 5-10% faster than the HD5850 at your resolution.