You also may need to be careful with buying a workstation, because you may well need an operating system and applications that can take advantage of the extra horsepower. You may need 64-bit Xp or Vista, or multi-processor support etc. And then, the software a home user commonly uses derives no benefit from these things anwyay.
Home software is on the verge of making great use of 64-bit archetecture, heavy RAM, and multiple processors, (these days, multiple cores) but it's just now starting. By the time it's fully mainstream, cheap consumer hardware will have left a used business workstation in the dust.
Also, legacy compatibility with certian business, science, or engineering functions may mean that a more "powerful" workstation might still be slower in some performance aspects as inexpensive commodity consumer PC hardware is. And it may suffer some of the same issues as server hardware, have weird hard disk arrangments, or expansion slots that are proprietary, or not what's popular now for video cards etc.
The analogy is that the surplus Army tank, or Deuce-and-a-half is durable, got tons of torque, and is literaly bombproof, but the cheapest Korean sub-compact new off the lot will have better fuel economy, is allowed on all roads, and in parking structures/shopping malls, has insanely better 0-60, but will crumple like Kleenex in a crash.