Going the Intel route would not be wise as the new athlon 64s beat the intels even in the multimedia department, plus Intels cost like twice as much and run very hot. AMD is the way to go. Also your 939 board is very future proof, so in a year or two you will be able to pop in a dual core CPU when they come out and get wicked performance. I think that you will be better off running 2 gigs of ram in dual channel mode over 3 gigs single, because dual channel is faster plus I can almost say for sure you will not use all 3 gigs of ram.
For a s939 board, the MSI K8N NEO2 PLATINUM would be a very good choice for price / performance