I thought that was the cheap version of the 5850? Am I remembering correctly?
It was the cheap one, but it wasn't really gimped (maybe just a bit in terms of overclocking).
Assuming the GTX460 can get to about 850/1000 and the 5850 Xtreme can get to about 850/1200, the 5850 is a bit faster. At the clock speeds the OP suggested, the GTX460 is just barely faster than the stock 5850.
The GTX460 at 850/1000 (i.e., the GTX560) is about the speed of a 6870, and the 6870 is about 7-10% faster than a stock 5850. So a GTX460 at 800 would be maybe 2-3% faster than a stock 5850. Again, overclocked at 850/1200, the 5850 is the stronger card, just beating a 6870.
Between the two, I'd choose the 5850 for its efficiency, but I agree, it's easier to buy a GTX460 if you're looking to go with dual cards.