Well, virtualization requires lots of RAM and fast HDDs. I recommend you build a system with at least 4GB of RAM (that would hold fine two virtual machines inside a linux or W7) or more and a fast HDD like the WD black or Spinpoint F3. You may want to RAID 0 two of them for better performance. Keep the other one as back-up.
Now if you find the exact HDD model you have,, you could get another of that type and RAID them, but only if it's not an old model.
For the CPU, a quad core would do...almost any Phenom since it won't make a too big difference having specific models. Even the older 9550 is fine.
The memory...make sure it's 1600MHz with CL8 if you will use DDR3.
World's best Red Alert 2: Yuri's Revenge mod and Star Trek: Starfleet Command 3 mod: Overmind.ro