680sli? Darn. Boards based on that chipset tend to be poor overclockers though you should see how well your particular board overclocks. If it supports 45nm Core 2 Quad CPUs then you may want to get one on eBay or something, but only if your board can do a decent overclock on it.
If you can though it's best to upgrade your Motherboard, RAM, and CPU. I'd say getting a decent P67 or z68 board with an i-3 for now that you can upgrade latter. Of course if you can afford to go with a 2500k then do it