Tom’s just posted an article on the benefit of going 4 cores vs. 2 cores. Interesting read, however since the readership is heavily biased toward gaming, its testing apps were mostly games that have not been written to take advantage of the 2 additional cores.
As far as using quad core for virtualization however, the advantage is obvious. Of course, performance is more dependent on the virtualization architecture than a few mhz in speed, as can be seen here. If performance is a concern, then be prepared to throw more money on hardware, or stay with paravirtualized solutions like Xen. My personal recommendation is OpenVZ / Virtuozzo for best performance as they are OS based solutions. For manageability though, VMWare currently reigns king. Stick with VMWare server if you want free, but keep in mind that performance will suffer.
Or I guess you can just throw more hardware at the problem like always. Thanks, AMD & Intel for the price drops! Please keep them coming!