Music:
I'm not sure what your musical tastes are, but if you're interested in high quality legal downloads,
http://www.bleep.com/ sells individual songs and full albums encoded at 320kbps (very high quality) in mp3 format and without DRM (i.e. you're not limited as to what you can do with the tracks you buy). I've only actually purchased one album from them so far, but was pleased with all aspects of their service.
As for CDs,
http://www.cduniverse.com/ is probably one of the better online retailers.
If I could afford it, I'd buy a pair of high-end Sennheiser headphones:
http://www.sennheiser.com/sennheiser...hones_high-end
As for TV tuners, since it's for a laptop you'll have to go with an external unit, only possible issue I can see here is if your laptop is old and only has a USB 1.1 port (unlikely).
I'm by no means an expert on audio mixing, so I don't know if it will be too basic for the stuff you want to do, but
http://audacity.sourceforge.net/ is a nice piece of open-source audio editing and mixing software. As I said, I've only used it for very basic procedures, but it's worth a look.
I think this was already mentioned, but if you plan on any fast-paced gaming it's best to go with a corded mouse (better response time). Logitech is a good brand, I use an old MX518. I'd probably go with one of their laser mice, perhaps a G5:
http://www.logitech.com/index.cfm/mi...5&cl=us,en
Speakers, Altec Lansing is a good solid brand. One recommendation is, if you're considering a surround system, make sure that a) you'll be able to set them up effectively (I have plenty of space, but the layout of my computer room means that there's really no good place for the rear speakers), and b) you wouldn't be better off getting a higher-end stereo setup for the same price.
Hope some of that helped.