I would say push a little further and get a GTX 460. It's a bit more money, but a year down the line you'll be glad that you did. Especially if you end up getting a bigger monitor or want to cover yourself for furture game releases.
GTS 450. The GTS 250 is actually a little faster but I'm sure with driver optimizations more will be squeezed out of the 450 till it's the other way around. Not only that but with the 450 you get DX11. I think a GTX 460 is a bit overkill for 1440x900 but if you want to enjoy the higher settings in more demanding games then the 460 is worth the extra money.