well the 3Dmark06 scores say alot,
I've seen a m9700 pull about 5500, and a 9750 about 9000. around there.
But the main reason would be the dual core processor, alot more future proof and ALOT faster than the single core found in the 9700. If you're planning on keeping it for a long time and running vista the dual core will keep you going for a long time and will run everything alot better. But if you want you can wait until the 7950 has DX10 cards in it so you wont have to upgrade. I don't know when that will be though :/
My 9700 runs FEAR and Oblivion just fine with pretty high settings, It runs Battlefield 2 With settings maxed out great. But I'm sure as games become multi-threaded (you need a dual core processor to run correctly) You're gonna need the better processor in the 9750, and the GPU's too.