That's not a bad GPU for that price, but if you can stretch your budget to $220, this GPU would rape it and this card OC's well to. Plus Nvidia cards have PhysX that AMD cards do not. It will use a bit more juice though.
That's true, but having PhysX for the upcomming Unreal Engine 4 games and Batman, Metro 2033 with PhysX does make a big difference. Also the GTX 480 is great at folding if you do such a thing as Nvidia's Cuda cores are better overall then AMD.
Since you are on a 450W unit, I would suggest this then.