First, you probably can't get a motherboard for under $40. Such a board would undoubtedly be dead on arrival.
Second, if you want to be able to handle your older memory and still be able to go with newer memory later, I recommend this
board. Granted it doesn't support as much overall memory, but it does handle both DDR and DDR2.
As for the processor, there is no reason to hold back and stay with single core. The extra multitasking with dual core is worth it even if the programs couldn't make use of it.