totally get a soundcard. i have the x-fi right now. makes my crappy speakers sound so much better. of course you won't know what your missing unless you've ever used one. IMO you should split your extra 400 dollars on 100 on speakers, get the audigy2ZS. a good card. which is around 70. and that leaves you 230 on a monitor.
well we were going to give my sis the old computer and im using the same case my bro gave me becasue i like it (not the preetyiest thing in the world) but its good quality. I dont know of anyplace that you can sell your old parts exept maby ebay.