I took a look at Micro$oft to see what they have to say. Basically here's a quote of how their licensing works.

*For most currently shipping Microsoft software with processor limits, each processor counts as a single processor regardless of the number of cores and/or threads that the processor contains. For example, Windows Server 2003 Standard Edition can be used on a four-processor system, whether the processors in the system are single-core, hyperthreaded, or multicore.

This would indicate that there is a difference between the Intel Quad Core and the AMD Quad Core. The Intel is 4 cores on one physical chip but the AMD is 2 dual cores on 2 chips. From an OS standpoint you should be able to run 8 Intel cores (on 2 chips) but only 4 AMD cores (also on 2 chips).