Did you by chance mean overclock instead of unlock? People have overclocked these cards and gotten to (or close to) stock 5850 performance numbers. But as far as how the 5830, 5850, and 5870 technically use the same core GPU with some units disabled in the 30 & 50, there's no way to re-activate the disabled units within them. (Hence my reference to Phenom II CPUs.)
At that resolution, a 5750 or 5770 would be a very solid performer. No need to spend the extra duckets on a 5830... Honestly, the 5830 is just a price-gap filler card. If you really want something powerful, you'd probably be better off continuing to save cash for a 5850. My opinion, of course. Hopefully you'll get a few more to help you make a decision.