I really think the 5770 is a much, much better option.
The GTS250 is another of Nvidia's rebranding nonsense, ur essentially buying urself a 8800 ---> 9800 ----> GTS 250. The card is slightly different from an 8800GT, but the performance is pretty much the same.
In my opinion (and experience) a 4850 would perform a lot better than a GTS250. And the 5770 is comparable in performance to the 4870, so I think it's leagues ahead of the GTS250. Not to mention that it has DX11, and new die size means it uses less power and produces less heat than the older generation of cards.
It really will be a much better choice for you, especially since you don't upgrade often, and will definitely be worth the R500 premium.
And no everyone, I'm not an ATI fanboy, I have an Nvidia card. But the fact is right now the ATI stuff is a much more attractive option. For your 500 bucks you're getting brand new technology, a card which most likely won't be obsolete for a few years due to DX11, and much better performance and efficiency over the GTS250.
Bottom line, with the 5770 you're getting technology a few months old, with the GTS250 you're basically getting something 3 years old now.