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 It's a pain in the ***, expensive even now, and when it's time to sell your just going to be stuck with 2 outdated cards to sell instead of 1.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.
Haha, don't have the cash for the 260. I don't want to spend over 160 right now. The 8800s wont be told for awhile I don't think? MY 8600 runs UT3, BF2, and M2TW all on the highest graphics still, and an 8800 is an upgrade. My plan was to get an 8800 now, and then around October get a 2nd one after price reductions and as new games are coming out. You still think the same?
edit: no idea how to OC my cpu, otherwise I'd like to get it to like 2.4 ghz.