If you want good quality sound output then go for a soundcard IMO.
Fair enough some of the higher quality mother boards will have decent on board sound quality, how ever in a prebuilt mass manufacture PC i would doubt highly that the sound will be any good.
I would reccomend:
http://www.overclockers.co.uk/showpr...d=1078&subcat=
I know you said you want say "prebuilt" but these guys offer a great service from my experience any way, and you will get a better PC for the money, as well as still getting a 12 month return to base warrenty.
Since its under 400 you would still be able to buy a good soundcard and install it yourself, or buy one from them and they will install it when they build it.
It doesnt have a 1tb hard drive but im sure they will install one in place of the 500 if you request for a small fee, or you could always install your own (piss easy).
This is the best PC (along with good SQ) you will get for under 500 i believe.
Hope this helps (dont be scared that its not from a highstreet store, they have been going for ages).
EDIT: Didnt notice above, if you are not scared to build the PC (dont be its easy), then go with the above! Otherwise what i have posted is your best option IMO.