Actually the
5100 MCH supports Registered DDR2, but lacks support for the 1600FSB used by some Harpertown models.
It SHOULD support 1333FSB Harpertown parts, we use a pair of 3.16GHz, 1333FSB chips in a 5000X based board with the latest firmware for our lab server, but don't take my word as gospel on that.
Personally I'd take a Core 2 Quad for gaming, you save on power and games will almost universally prefer lower latency, high speed memory over RDDR-667 or 800MHz FB-DIMMs.
Thats not entirely true. Some games are able to benefit from quad-core. The latest build of Company of Heroes sees improvements to the MINIMUM frame rate, taking it from single figures on a dual-core to around 30fps on the DirectX 10 codepath, Unreal Tournament 3 scales reasonably well dual to quad, the latest Enemy Territory Quake Wars patch introduces a multi-threaded renderer along with the separation of the 'server' and client even in single player mode (as was used in Quake 4), and there are various other examples.
Of all of them actually CoH is the best, taking it from choppy to smooth motion, the rest tend to be increases over already playable levels of performance.