If you can find a sub with at LEAST an 8.5 inch speaker (10 would be preferable) AND an adjustable crossover, you could definitely improve the situation. You want a speaker large enough to actually be able to reproduce the lower frequencies you're needing without simply creating a nasty sounding resonator box like a lot of the cheap subs with small speakers do. The adjustable crossover will let you dictate at which frequency point the lows are sent to the sub and everything above that point, sent to the main speakers. This will allow you to dial in the best balance between your sub and your speakers, rather than letting the sub dictate that for you, since the sub is not really in a position to know what sounds best for your application.
If you get a sub with a crossover, I'd set it somewhere right above 80 hz as a starting point, and just adjust up or down and listen to how it affects your speakers. Allowing the sub frequencies to be routed to your sub takes a lot of stress off your desktop speakers and lets them open up. You'll hear a big improvement on them.
Secondly, resist the urge to drive the sub hard. You want balance, not boom. The whole goal is to let the whole frequency range be represented by extending the frequency reproductive capability below what your desktop speakers are capable of reproducing, so with that in mind, make your decision based on that ability...
Which means you need a speaker large enough to reproduce those low frequencies (not necessarily amped enough to push them hard) and the ability to adjust the crossover point at which certain frequencies will be handled by the sub, and the rest by your speakers.
Hope that helps a little.
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