OK, here are some suggestions, but I do not have the time to get links.
If you are trying to save money, forget the SLI mobo and get the DFI Ultra-d, one of the best socket 939 mobos.
Like said below, go for the 6800GT over the ATI (make sure its PCI-e!)
Yes, add RAM
You need DDR400 (also called PC3200) Probably 1gig (2x512)
You do NOT need the sound card, the PCI-USB adapter, or the network card. The sound on the mobo will be as good as a $50 sound card, and you will have plenty of USB ports and a network port on the mobo, no worries there.
Get the Athlon 64 3700+ San Diego over that 3800+. The San Diego is a better core.
Oh, and I would look for another case
and get a GOOD PSU.