I run about 3-4 different VM's on a dual core with 4 gigs of ram on any given day. Overclocking is a great way to get more for your money. Of course your mileage may vary.
A good Gigabyte board some G.Skill ram, hardy PSU, a hefty sized heatsink and a case with open airflow and your set. The e6750 (stock 2.66) was able to do 3.3 without breaking a sweat. I did some research in overclocking forums and simply put together the same system others had with experience.
This was a year ago, but am happy to say the system has been working great. Not sure about what the best price for overclocking hardware is today, but it may be something to check into if you are looking to get faster performance for less money.