For 1400x900 a GTX570 is overkill but I would say go ahead and get one because you will be able to max out games for a while.Also either the GTX560ti or 6950 will be able to max out games at that resolution.
The GTX560ti and 6950 perform about the same the only difference between them is which game you play because Nvidia cards and AMD cards perform differently in each game.But it's usually only a 5-10fps gain which isn't significant.
This is cheapest GTX570 I could find for Canada.It's not bad but it doesn't have very good cooling.But it's still very worth the extra $20.
I would get the GTX 570 but I'm hoping around Black Friday it will go down significantly in price. I've heard that the 570 gets way too hot as well so I am concerned about getting a model that only has one.