Just clearing things up, there's no motherboard out currently that supports the AM3 socket. Right now it's AM2 and AM2+ for AMD. (I just noticed the irony behind the socket naming)
The GA-MA790X-DS4 is a AM2/AM2+ motherboard and will work with all Phenom CPU's out currently. You can use a AM2 or a AM2+ CPU in that motherboard but when an AM3 CPU comes out you should still be able to use a AM3 CPU in that motherboard.
AM3 CPU's will have two pins less than a AM2+ socket so it will fit, minus two pins that would be on a AM2+ CPU.
As for an upgrade, I would really wait and save up. Next month AMD is releasing there new line of CPU's on the AM3 socket with DDR3 capabilities. Not only will this increase the Memory preformance but also the uncore(memory controller + L3 cache).
If you look at the reviews on the Phenom II CPU's out right now, they are fighting competitively with the mid-range Intel Quad Cores. This release should bump up the new Quads to fight with the Higher range Quad Intel cores.
So - Wait for next month and look at some upgrade options that are right for you. DDR3 is a good performance factor in the new setup as well as a faster uncore(AM2+ CPU's on the Deneb core are set to 1.8Ghz while the AM3 are going to be set at 2.0Ghz)
