Unless you are needing full duplex, onboard sound should be just fine in any modern motherboard.
The only problem I have ever encountered is trying to record in fraps while playing. For that I needed to throw in my old SB Live card. Other then that, onboard audio is very adequate for the majority of applications, and most certainly for Mame dedicated boxes.
Personally on your setup above, I would cut the ram to a gig, and get a core 2 duo. The performance is well worth the price difference. And you can save a little on the ram, Mame shouldnt need 2gigs, and you shouldnt be running much overhead either.