Short answer: I'd go with the first one. Bigger hard drive, 512 MB video card as opposed to the 256 of the other one. Only thing is the first one is an ATI vs the Nvidia 8600 which I know is a very good model.
Long: I'm curious as to why you're looking at best buy and gateway in particular. Best buy doesn't have great prices, and all the gateways I've owned in the past were junk. Newegg has a few, a couple don't come with monitors but they are significantly less if you're dead set on a Gateway..
http://www.newegg.com/Product/ProductLi ... me=Gateway
Or I would just configure a Dell XPS, they're nice computers and you'd probably get something comperable for less money.
http://www.dell.com/content/products/pr ... l=en&s=dhs