I'd say get a HD7970 and overclock it to a GHz Edition or get a GTX670 and overclock it to a GTX680. but looking at W1zzards latest review Club3d HD 7870 jokerCard the HD 7970 GHz beats the GTX680 in most benches... But not by much and remember both top cards can arguable be the best if someone overclocks them... You can't go wrong with either card.. So, I'd get the cheapest with the most Vram and right now that card is the HD 7970 @ $379.99
with a $20 MIR which brings it to $359.99.