What is kinda nuts is that this board:
http://www.msicomputer.com/Newsrelease/nForce.htm
has one of the best sound setups ever made for a computer. I think the a7n266 might have the a Dolby Digital 5.1 real-time encoder, but I don't see them say that anywhere, so I'd go for msi's board till more info is known. I am not sure about plugging computers into synthesizers and other instruments, sorry. Find out about the new SB live sound card they're makin (or anybody know about it?!?!) before you but the nforce board, that might hook into those instruments.
Don't spend the extra money for a laptop. All the components above should do very nicely, except the only part I am unsure about is that cheap toolless case.
While way more expensive,
www.coolcases.com has the best air cooled cases and this is to many people the best mid tower:
(Modified) Lite-On Fs020
http://www.coolcases.com/catalog/p16.html
This place has the best power supplies (don't get their ultra quiet models because they don't move any air):
www.pcpowercooling.com
but that place is very expensive and I think the Enermax 465W EG465P-FC is a better deal for like $85. If you want a cheap no name power supply, just get the heaviest one you can find as that is a fairly good way to judge supprisingly.
For music software I am not to up on it, but try finale 2002 and cool edit. use www.google.com to search for it. oh I'll do it...
http://www.codamusic.com/coda/
http://www.syntrillium.com/
someone recommend stuff to hook those synthesizers up to the computer. I don't know anything about that.