For those of you looking to upgrade your Intel Socket 775 Systems before the processors go the way of the Dinosaurs.  I have come up with some recommendations based on the availability of the processors and performance.  I would spend the money now and go with one of the Core2 Quad processors.  It's going to run you a minimum of $225 for the quad or go with one of the Higher End Core2Duo processors like the E8400 or E8500.  The processors listed here have the Intel Virtualization Technology (VT-x), which is what you need if you want to use a virutual machine with out any major Lagg.

This Chart only refers to the recommended processors.

Here are the processors I would recommend to you.

Core2 Quad Processors

Q9400 / Q9400S / Q9550 / Q9550S / Q9650 / QX9650 - These all have a minimum of 6MB of Cache and supports Intel® VT-x.

If you want more detailed information about the Core 2 Quad Processors you can compare them here

You can purchase the Core2 Quads From B-Com or Memory Express

Core2 Processors

E7600 / E8400 / E8500 - These all have a minimum of 3MB of Cache and supports Intel® VT-x.

If you want more detailed information about the Core 2 Duo Processors here

You can purchase the Core2 Processors from  B-Com or Memory Express

Intel Dual Core Pentium

I do not generally recommend the Pentium Processors.

If you are insistent on a dual core Pentium; I would pick the

E6300 / E6500 / E6600. - These all have a minimum of 2MB of Cache and supports Intel® VT-x.

If you want more detailed information about the Dual Core Pentium Processors here

You can purchase the Dual Core Pentiums from  B-Com or Memory Express

For benchmarks you can read this PC Perspective Article

For more information on about Socket 775 Processors with the VT-x Technology you can visit http://ark.intel.com/VTList.aspx