AFAIK (remember I'm a brit) $500 will get you a fairly good sub.
But it'd be a complete waste (along with your Bose speakers) to use it on a) a rubbishy soundcard and b) a system without an external amp.
Grab yourself an Audigy Platinum (or PLatinum eX) - WAY better drivers than the Audigy 2 ATM - which will come with a breakout box which'll give you a decent optical or coax SPDIF out. Plug this into your amp, and hang your speakers offof that.
If you can't be bothered with an external amp, just sell your Bose and grab a set of Cambridge/Creative PC speakers. They'll sound just as good, since built in amps in soundcards have a terrible SNR (signal-to-noise ratio).
As an aside, the "wattage" of speakers is a stupid measurement designed to impress the gullible. NO speaker outside of a concert hall actually delivers 80 Watts, this "80 Watt" is measured on a scale called "Peak Music Power Output" or PMPO, which is another term for "make up any number you want". 80 W is actually powerful enough a speaker tyo make your eardrums bleed, if the watage is measure properly. The only
real measure of a speakers power is the RMS Wattage. Even so, buying a speaker buy their wattage is stupid. What is the distortion level at any given wattage? What is the SNR? These are the things that govern quality. My speakers are 6W each, with a 10W sub, and with the dial turned up to 2 they make the floor rumble. They sound pretty good too (although I don't have an external amp yet)
