If you can spring the extra cash, the HD5770 beats them all. Though it's overkill for your resolution..
If you're planning on upgrading your monitor soon I'd go with the 1Gb GTS450. However if you're keeping your 19'' monitor for quite some time, you won't notice too much of a difference with the GTS450 and GTS250; get whichever's cheapest. Either card will play every game your resolution quite comfortably.
Alternatively look for the HD5750. If it's cheaper than the GTS450 then it too is also a good buy.