It will only be decent sounding if a computerized sound is what you're after. I tried this using many programs/samples etc when I first started. Basically the best bass sounds I could get either sounded like techno, or a stupid piano. The bass guitar is an important instrument, and can't be faked. You wouldn't try replacing regular guitar with a computer!
Buy a bass and a DI box if you want traditional, realistic sounding bass. If you're into hip hop or experimental shit, then a bass patch might suffice.
I'm not anti-fake-sounds, I'm just trying to save you some trouble. Always be sure to experiment if you're feeling inspired! I love mixing electronic crap with more traditional music. There is some great software out there....if you're interested, head on over to harmony-central.com and check out their software links.
Slackmaster 2000