First is this just to blast the tunes, or create the tunes?
If just to blast em, go with a entry level X-fi, sound card, otherwise if you are producing, take the hit on the cash for the top of the line X-Fi (it's about 550 Cnd). As for everything else, I'd go with a middle of the pack mobo and AMD cpu, and an ATI X800GTO or similar card.
As for speakers:
If you want sheer volume, enough to make people on the next block over call the cops, go with the Logitec Z5500 speakers 1005 watts total power, and some great connectivity as well. Also they have a DTS2 decoder built right into them.
If you want picturesque sound quality at the loss of volume go with a set of klipchs pro media speakers.
Or if you are a little electronically inclined, and want the hottest/loudest setup, purchase a receiver and buy the speakers yourself. A note here, if you want some GREAT sounding speakers, look at polk audio.
That's my $0.02