For single core, the processor M2 recommended is the way to go. Extra $50-$150, gets you a dual core. If you decide to go the ASRock Dual route then you need 939 processor now & DDR Ram. When you buy the daughter card later on, then you would need to buy an AM2 processor & DDR2 Ram. Can't mix & match the ram with the different sockets.
If you wanted to go AM2 & DDR2 right now, then using the ASRock Dual isn't best way to go. Better off getting a AM2 board with AM2 CPU, DDR2 ram, & decent PCI-E card.
Yeah more specs on your power supply & ram would help.