The E6600 outperforms every CPU that AMD have!
It consumes less energy than the X2 5000 and dissipates less heat and it overclocks much more. The E6600 is better choice than X2 5000+ in all aspects.
The E6600 performance is almost not afected by memory latencies and performs almost same with DDR2-667 and DDR2-800. It has very nice features and technologies that are much more efficient than the IMC on the X2 CPUs: memory disambiguation, excelent prefetchers and large shared L2 cache.
On the other side, the sAM2 X2 5000+ needs low latency(CL4 or less) DDR2-800 inorder to perfrom same as the s939 with DDR-400 CL2. With DDR2-667 the sAM2 will perform slower than the same clocked s939 with DDR-400.
Like other discutants said, most of the Core2 mainboards with i975 and i965 will support Core2 Quad. Also other chipsets will support C2Q, you have to look after mainboards with VRM 1.1.
All the sAM2 mainboards will support futre sAM2+ and sAM3 CPUs with the K8L architecture. But these CPUs will not be available until Q3 2007.
The s939 mainboards are going to die together with the s939 90nm CPUs.