I'd say go with the 750i if you can, I don't know if you plan on upgrading in the next 6 months, but it will give you MUCH more room to play around with
As for the 650i, each PCI-E lane is only 8x,
so you will not be able to get 100% of your 9600GT's SLi'd.
I'd say spend a little more cash on the mobo. And I thought you were gonna be using a TRUE? No more quiet PC? If you can, get the xigmatek heatsink and slap on a 40 CFM slipstream. won't overclock much, but will be 10x quieter then the stock HS, even on an overclock.