The non-reference cards cost a bit more ($20 typical), but have a lower cost per frame. In other words, you get more bang for the buck.
Guru3D uses the following games in their test suite: Hard Reset, COD-MW2, Far Cry 2, ANNO 1404, Metro 2033, ANNO 2070, BFBC2, BF3, Crysis 2, AvP, Lost Planet 2. Total fps (summing fps in each game @ 1920 x 1200) are tabulated below along with their cost in dollars per frame @ 1920 x 1200:
670 DCII gets 999 fps for $420 at a cost of $0.42 per frame
670 gets 917 fps for $400 at a cost of $0.44 per frame
680 DCII gets 1077 fps for $520 at a cost of $0.48 per frame
680 gets 989 fps for $500 at a cost of $0.51 per frame
7970 DCII gets 924 fps for $415 at a cost of $0.45 per frame
7970 gets 872 fps for $420 at a cost of $0.48 per frame
Note that the Asus non-reference DCII has a 2 -3 cent per frame cost advantage, getting you about 4 - 6% more for ya money.
I'd normally suggest ya be a good bro loan him the $20
but if ya can score a deal for $340, there's no way i could argue against that.