If you have a good motherboard you could drop in a 45nm Core 2 Quad and OC it assuming you also get a good cooler. If you overclock your current CPU then a GTX 560 or Radeon 6870 are probably the fastest cards that make any sense if you're gaming at 1080p or at least 1680x1050. If your resolution is lower then even those would be held back a bit too much by your CPU in general to justify not going with a cheaper card.
I'd say go for it. You'll be throttling the 580. Then when you upgrade your mobo and CPU you'll get the benefits of the new cpu and the unleashing of the 580. If you get a 560 or something then the moment you upgrade your CPU you'll be throttled by your GPU.
the only thing good about that rig is the 8 gigs of ddr2, seriously, if you can get your hands on a c2q for cheap, you can throw a gtx 560/6950 in that bad boi and game away, other than that, full system upgrade
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