Yes, you can SLI two different brands' cards as long as they're both the same model (i.e. 670s).
HOWEVER http://www.newegg.com/Product/Product.aspx?Item=N82E16814127685&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID= Look here. According to the rebate date, this deal ends today. It's $70 cheaper than the Gigabyte one according to Pcpartpicker.