Well the Sandybridge Mother boards are hitting retail now on new egg and other sites.1155 systems are acording to intel supposed to be compatable with Ivybridge down the road. So you could build an 1155 2500k or 2600k system (k) if you want to over clock. Also the P67 system for overclocking the h67 if your going to utilize the onboard video Thats if you want to buy now . If you want to wait a bit for the z68 motherboards they pretty much incorprate what the h67 and p67 do in one board. Or if you want to do the extreme end wait till fall for the 2011 chipsets. The 2011 chipset will be the extrem end like 1366 was as I think they will not come out with the 1365 chipset that was supposed to be a direct 1366 replacement.
So it comes down to how long you want to wait.
You can start now
Your can wait for Z68
You can wait till 4th quarter and see if the 2011 chipset is out yet.
If you do buy now make sure you get B3 stepping on a 1155 board.
Depends on budget also.
Those charts tell me your C2Q 9650 is not yet ready to be out to pasture.
Have you thought about OC'ing your C2Q 9650? With the right motherboard and cooling something close to 4Ghz should be possible.
Picking a good cooler, one with socket 775 & 1155 compatibility would mean you could move it forward with your GTX 580 when you do get around to upgrading the whole system.