GeForce GT 420 =/= Radeon HD Mobility 5870.
A Radeon HD Mobility 5870 is equivalent to a GeForce GTX 470.
@OP: Just checking, you're not talking about a laptop, right?
If you're talking about a desktop computer, I'd personally recommend going with any of these here. http://www.newegg.com/Product/ProductLi ... gesize=100
You pretty much can't go wrong with them. Obviously, the 760 will perform better than the 660, but I'm not sure of your budget (50 dollars is the difference though, and if you can do it go for the 760.) Of course, if you think you can hold out for another bit, the 800 series will be releasing sometime within the next three months.
a Mobility Radeon HD 5870 is more like at GTS 450 desktop card, than a GTX 470 card, most of you guys using laptops should buy a desktop pc, or stop trying to play games on your low end laptops