You should OC your cpu if your going to replace your gpu. If you don't OC your cpu when you get the 9600gt/8800gt then you will have a tiny bit of bottleneck.
I recommend you get a 4850 and forget about SLI. It's a pain in the ***, expensive even now, and when it's time to sell your just going to be stuck with 2 outdated cards to sell instead of 1.
I've done SLI, I've done single cards, I've upgraded just like your about to. Trust me, save yourself the trouble and just get a single 4850. And it WILL work on your motherboard, you don't need an "ATI" mobo.
If you don't listen to me, have fun with a generation or so old sli.
edit: Also this is a slightly different situation then people who ALREADY OWN an 8800 gt/9600gt. If you are REALLY an nvidia fan boy, get a gtx 260. Thats more power then most people need.