Call IBM and Georgia tech and see if they'll let you buy their 500 GHz CPU with cryogenic freezer.
But seriously, it really depends on what you need. 20 CPUs would do absolutely no good for games, and , if you're just looking for a fast computer, after about 4 you'd see absolutely no difference. More than 8 CPUs/cores is really only for servers and other mainfraims with high traffic or for scientific calculations and computing. Just getting it for the heck of it is what I call a "Money Rig" (A machine designed with ego in mind, provides no more benefit than a machine of substancially less money for the user's particular needs).
Not to mention, if you get this, you'll need specific processors and chipsets which are designed for this kind of processing in mind. (You can't have 20 Pentium Ds, or use a 975X chipset). A motherboard with a chipset complex for this would be several thousand dollars. And the CPUs, well, they're not going to cost as much as your little Pentium 4 or Xeon, you'd be looking at something like Itaniums, which cost several thousand per CPU.
You'd also need a very expensive OS, such as Windows Advanced server, not just server, advanced server edition, which can only be obtained through Microsoft IIRC, VIA the VLK (I may be wrong here), or a custom compiled linux kernel (not easy), if not, the rest of the power would go to waste, as if it wouldn't with those anyway, since you can't possibly use that kind of power. Not much software would actually be written to use that power, especially not what we all use. This machine will not power an Xp install any more than a Quad Opteron setup. Not to mention it likely would be running a different architecture, PowerPC or IA-32.
If you're still really intrested in that, as mentioned above, contact Sun Microsystems or something. Even Dell possibly. What I reccomend, is either setting up a Quad CPU dual core Opteron system, with 16G of RAM, or even better, a nice dual core CPU FX or Conroe system, with SLI, PhysX, Audigy, dual DVI monitors and all the bells and whistles that would probably rock the house, and you'd be more than happy with. And save a lot of that money. It'd run all your software plenty fast as well. I'd be glad to assist you in configuring a very nice system for all of that, and to me, that's the most logical route.
