My AMD rig just recorded some maxed-out BF3 stuff, but it's running at 1680 x 1050 and the recordings themselves were "1/2 frame". (Half size)
My phenom X6 @ 3.4Ghz (with 2 560's in SLI) was able to play the game maxed out pretty well, but on a few occasions it struggled once I started capturing. My i7 rig did to,o but only about 2/10 of the times I started recording and the AMD rig bogged about 4/10 times.
The 1/2 frame video quality wasn't bad though:
This is how it turned out when I did the same experiment with Crysis 2:http://www.youtube.com/watch?v=vbGUfaPg-Vo
(Neither one of the rigs had much trouble recording this one.)