for the cpu, it follow the motherboard whether the motherboeard is support it or not. but for that ddr2 to ddr3 socket motherboard. trust me, wont work at all (maybe not even fitted)
but, in case you havent brought any mb yet, recommend ASRock G41C-GS cuz it support ddr2 and ddr3 with intel chipset G41 (separate slot. cannot combined ddr2 and ddr3 at same time)http://www.asrock.com/mb/Intel/G41C-GS/
since my mb is N68C-GS FX which support DDR2 and DDR3 (AMD only).