I'm upgrading my GPU and was originally going to get a 7970 ghz and OC it but my current card sold for more than I thought it would so now I have enough to get a second card. Is one better than another; 2x 680's in SLI vs 2x 7970's in Xfire? Since I'm getting 2, I probably won't OC. I'll have roughly $900 not including shipping and tax. Any advice / input is greatly appreciated. Thanks!