If it's a Model M keyboard, that's definitely a keeper, or at least good for eBay. But the computer might as well be a doorstop.
No, no, definitely not a Packard Bell.
Oh, it's not that simple either... If you've got a random assortment of parts, there's all kinds of wacky little obscure compatibility issues that might pop up - and that's assuming all your parts are completely functional to begin with, and they might very not be.
The first example that comes to mind is RAM - you can't just toss any old SIMMs into your available slots willy-nilly; some of the motherboards were mighty particular about exactly what went where.