Last month Intel released their new Nehalem processors, which knock the socks off their previous generation processors and now beat AMD for dual socket servers. For years VMware has published benchmarks for various servers so you can judge their virtualization performance.
The sweet spot for virtualization is a dual processor server. Why? Quad socket servers generally cost more than 2x of a dual socket server. With the new Intel Nehalem processors, you can now get quad socket performance at dual socket prices. Sweet!
According to the latest benchmarks, for an 8 core system the HP BL490c G6 is the new leader at 24.24 @ 17 tiles. In fact, it beats the current 16 core leader which is rated at 20.50 @ 14 tiles.
You can see all the latest results here. The full disclosure report can is here. If you really want quad socket servers, then AMD is still the leader of the pack. But that could well change with the Intel Nehalem EX chips are released in early 2010 which will be 8 core and support four sockets.