i'd suggest an 8800 gts -320- instead of a -640- ... unless you have a huge screen you're not gonna need it, and for the cost just wait 90 days to upgrade to an 8800 gtx (or better!)
i was told the 6420 has better performance than the 6320. despite the product difference....
give building the computer yourself a shot. i had a bit of trouble connecting the 1394, etc, it was nerve wracking for me, and i didn't want to mess anything up, but you can do it
arrange the wires neatly. i couldn't tell you whether wires touching components will cause a meltdown or not, but from the looks of things (i backed down eventually) it's not a big deal
if you can afford it get a nice fan for your case. because some of them need to be installed/supported from behind the motherboard.
get yourself a sound card. dont' know if you've got one, but wow! (i've been playing guitar for about 7 years so i notice