You're a bench junky pos, get the 8800GT. Don't mess around. When both the 3870 and GT are oc'ed, the GT still pulls ahead. Just look at the scores in the 06 and Vantage threads.
I play CoD4 at 1920x1200 fully maxed out. It never goes below 48fps with FRAPS running. Oced to 700/1000 linked shaders. That's with stock volts and cooler. With better cooling (h2o, in my case) and the voltmods, I game at 800/1100 linked shaders, bench at 900-950/1100 linked.
It would take well over a Ghz from the 3870 to even come close.