300 is a really fair budget and you can go many ways. for example..
2.0, 2.1 or 5.1 computer speaker set.
2.0 powered speakers
2.0 powered speakers + powered subwoofer
2.0 passive speakers + amplifier
2.0 passive speakers + amplifier + powered subwoofer
(stereo receiver can be used instead of just an amplifier as well)
if you upped your budget to about 5, you could get into a 5.1 entry level hifi home theater.
hifi speakers will sound better than pc speakers as they are often better quality. i recommend going that route but some people like simple plug and play. also, if you want 5.1 for under 3 normally your options are limited to pc speakers unless you get a cheap htib which may not have the 5.1 inputs you want! (many are 2.0 input when cheap)
good powered speakers...
mackie cr3/cr4, maudio av30/av40, micca pb42x,
good passive speakers you an use with an amp or stereo receiver...
micca mb42x, pioneer bs22, klisch kb-15, sony speakers arent terrible
good powered subwoofers...
dayton sub-**** labeled products, polk psw10, perhaps monoprice
i'm running a klipsch 5.1 satellite setup (wish i could afford the nicer bookshelf options in 5.1) and i've been nothing but pleased. i like the treble detail the horns deliver. a pair of klipsch bookshelves with an amplifier may be hard on your budget but would be very nice as well. some studio monitors would also be good if you did not need much bass (you can add a sub if you want bass)