PCLab.pl has an interesting review and benchmarks of a 3.07GHz Westmere-EP processor (which is the six-core, 32nm successor to the Xeon 5500 series, Nehalem-EP processor). This unit has six cores, and hyper-threading for a total of 12 cores per physical processor. The L3 cache is bumped up to 12MB. This would give you 24 logical cores in a dual-socket system (with hyper-threading enabled).
These processors should drop right into a system that has Xeon 5500 series CPUs, probably with a BIOS upgrade. It usually does not make economic sense to upgrade processors on an existing server, but you should be able to if you need or want to. When these processors become available in 2010, there should be very little delay in them being available in existing server designs (such as the Dell PowerEdge R710), since all they should need is a BIOS update.