Comment 1st gen opterons (Score 1) 272
you said you're not in the market for server hardware, but have you considered 1 or 2 generation older stuff?
you can pick up single and dual socket motherboards for well under 200$. 2ghz chips for 50-100$ each(that's a pure guess, they may be even cheaper), 1gb ram sticks for about 10-15$ each.
I've got tyan s2895 and s2892 boards, both work very well with windows or linux, vmware esxi will install without any fuss if you use a sata drive, and, if you so desire, I hear they make a mean hackintosh. my s2895 has 2 2.6ghz chips and 4gb of ram, running about 6 (mostly idle) linux vm's that all feel as responsive as my desktop machine.
you can get opteron 240-246's or higher for pretty decent prices, possibly even dual cores.
the only downside is they run a bit warm.
that, or I'd suggest a core 2 quad. those can run a little warm too, in my experience. your ram and motherboard will be cheaper than the opterons, but I have no idea how they play with osx or vmware.