The computer as a part of the system doesn't mean the quality will be low. This is predicated first on the source content, which begs to be analyzed in your situation based on the comments above. Any fairly decent sound card or onboard sound should suffice with a good pair of speakers to provide you with quality audio. While your Creative sound card is designed for gaming it's a quality component. There are now cards specific to music but they will [probably] make no difference.
if I can use the computer to do something please share how
I think you already know this, but the computer will be the source of all your digital content. Your existing MP3s and ripped CD's (download and purchase dbpoweramp and rip everything to FLAC - or ALAC if you're an iDude) will be stored and served from the PC, as well as any of your favorite online subscription services (Pandora, Spotify, etc.). Whether you stay with powered speakers or migrate to a receiver and passive speakers, the computer will still play its part.
I might recommend starting off by locating a nearby Guitar Center and listening to a few pairs of powered monitors. If you hear something you like then buy them and keep the computer as the heart of your system. If nothing wows you, then start to audition passive speakers in your price range. I recommend being very critical in your passive speaker quest, as speakers sitting 3 feet from your ears, regardless how they sound from 8' away have the potential of driving you mad in no time at all.
As Arny said, you've got a heck of a budget, and with it a lot of choices. Auditioning is lots of fun, I'm envious! OK not really, I'm done for awhile.
Edit: As an afterthought, what kind of music do you listen to? Many monitors are deficient in the bass department, without looking I'm guessing you aren't getting much lower than 70Hz with the M-Audio speakers. Could this be the cause for your perceived mediocrity? To some people and some genres subwoofers make a big difference.