Soon, we'll see 150 MHz and 166 MHz versions. They cost only a little more than the slowest models, and deliver performance that is not much worse than that of the priciest chips. No products change as quickly as computers. The entire personal computer industry is only about a fifteen years old, but in those few short years the industry has progressed from the Stone Age to the Space Age. Considering the pace of this change, it just doesn't make sense to buy a computer today without planning for tommorrow.
Every computer claims to be upgradeable, but there are varying degrees of expandability.
The last two items require a bit of explanation. Even if you buy the fastest Pentium Pro system available today, at some point you're going to need to go to a faster processor. Some motherboards try to provide a way to add a faster processor later. The problem is, computer manufacturers don't really know what features computers will have two years from now. The best way to guarantee that you'll be able to upgrade your processor, therfore, is to make sure that you can replace the motherboard.