Computer speakers are really nothing like the quality you need. If you must have powered speakers there are some reasonably priced units around.
To monitor music on my computer I'm very happy with a pair of 1970's KEF medium price hifi speakers -- though the crossovers have been upgraded. Amplifier is a slightly newer vintage Mission Cyrus -- 50 watts RMS -- both were obtained at a flea market for the princely sum of £20 (roughly 30 bucks).
Thanks, fihart......i've definitely been looking for some bargains, and this is probably what i'll end up doing, buying some inexpensive used monitors. But the whole idea of actually building some custom gear really fascinates me of late, and i'm going to eventually get into doing just that. I found a plan online for a lo-fi 8-bit sampler, and i think i'm gonna start with either that or an 8-bit synth.