I'm not a gamer, But
(1) does he want to blow $100+ on just a CPU and Just how much of a gain will this provide in gaming. Seems that unless the CPU is bottlenecking the Current GPU (6850), then for GPU intensive games, probably not worth it.
(2) Also depends if the $100+ Cpu upgrade is worth interms of "time", ie when do you plan on complet upgrade?? If in the next 6 Monthes - forget it.
(3) A much better upgrade would be MB and CPU - the cost difference is about $126 -> 150. As HE could reuse all other components. Just the CPU ties him into a AMD system later, or accept the fact that he Just flushed $100+ down the Lew.
.. Reason for going the MB/CPU route is not just the CPU but to replace all the other dated chipsets. New MB adds USB 3, and SATA III. Not sure If current MB suports Xfire, but a new MB will support 2 GPUs @ (8x,8x) and He could buy that as an upgrade to improve gaming say 6 months after upgrading the MB/GPU. Note: USB3 and Sata III will not really improve gaming, BUT does improve overall Computer performance.
... While the AMD route is OK, My preference is the Intel SB route.
Added. Just ran across a very simular thread, Good read:
Intel Core i3 2100 Vs AMD Phenom X4 955 for GAMING.
http://www.tomshardware.com/forum/310141-28-intel-core-2100-phenom-gaming