Since all x86 CPU's are for the most part backwards compatible, whatever CPU you have now should work.

What I use for running legacy applications is Virtual PC 5 (hxxp://www.connectix.com/products/vpc5w.html) and install a DOS/Windows/Whatever virtual machine.

If you are running old games you may need a utility to "slow down" your machine to make them playable.

Hope this info helps

Regards

Perdition