I bought my soundcard (Creative Soundblaster Live! 5.1) for AU$50 about 1-2 years ago. They are pretty cheap.
Don't talk to me about cheap speakers, I've spent over $1200 on speakers and about that much on amplifiers, but in my opinion, don't go for 5.1 if you listen to a lot of music. It may be great for movies and games but pure stereo is the one for me.
When outputting stereo the sound card only outputs to the two front speakers anyway (not the central one), though you can have it set up to be 5.1 and to be honest I think it sounds a hell of a lot better than over stereo speakers, you may disagree with me but it all depends on what people prefer.
As for sound cards, I have a Creative X-Fi of which the sound quality is phenominal! Because this has just come out you can now get yourself an Audigy 4 for £40!, or you can get the Audigy 2 Platinum with controller unit for the same price, and the ZS version (has a much better controller unit than the platinum version) for £80, I'd grab the Platinum ZS so then you can also have optical sound output, DTS, Dolby II and more!
Now while Logitech's Speakers are undoubtedly the cream of the crop, there is just one thing to keep in mind, they're stupidly expensive but for obvious reasons, but what I'd do is not buy PC speakers but go for Entertainment Systems instead, they're cheaper, just as high a quality and they'll allow you to plug your entire home into them! Hell I've got my PS2, GameCube, PC, SNES, N64, and my PSP plugged into it via Coaxial, Optical, and lots of standard phono, couldn't get any crisper from my £70 Yamaha 5.1 Dolby Digital DTS speakers (rebranded as Pacific when ASDA were selling them). ^_^