Performance, power efficiency, and value seem to be best on the Haswell LGA1150 platform. Idle power consumption is low but the chips deliver top performance at various price levels.
If you don't have a discrete video card, then for under $300 you can go with:
Asrock H97M Pro4 motherboard and an i5-4590 CPU.
If you have a video card, you could move up to a Xeon E3 (has no onboard video) such as a Xeon E3-1231v3 for a total of about $325 (based on Newegg prices)
The H97 has limited SATA ports, so you may want to consider a server oriented motherboard such as Asrock E3C224 or Supermicro MBD-X10SLM-F-O. Lots of folks on these forums have put together systems using motherboards from both manufacturers. You can easily replace your mobo and cpu for under $500 and get a damn fine system.
All of the above though are DDR3 1600 systems. I suspect they will all work just fine with your existing memory, and probably to a level where you could never detect a difference between 1600 and 1333 MHz.
Final note, you can mess with this web page and filter out processors based on specific features you are looking for:
http://ark.intel.com/Search/Advanced?s=t
Once you have a motherboard targeted, do some google searches to see how much success folks have with virtualization.