The i7 extreme - is it the 990X that you're referring to? If so that's based on an aging architecture that Intel has announced is in the process of being discontinued. I think it's this:
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115079
When you get the 7970 - you will want a board that will support it out of the box with no hiccups. Part of that is due to the outdated Catalyst software (which AMD replaced today with a universal 12.1 driver) but right now the only CPU / board configuration that will support it is the i7-3930K. If you buy an SB build you'll need to wait until the Ivy Bridge chips are released - probably by this summer.
If you want to get something now that will support the 7970 out of the box you'll be looking at a configuration like this:
Motherboard:
http://www.newegg.com/Product/Product.aspx?Item=N82E16813121534
CPU: http://www.newegg.com/Product/Product.aspx?Item=N82E16819116492