To augment the discussion,
Your CPU is slow.
The real issue is this: your GTX570 simply does not have enough RAM for most modern games at 2560x1600/1440. I traded mine (exact same card, which I loved!) straight up for an HD6950 2GB, and then bought a second. This setup is both faster and cheaper than a GTX680.
However, that GTX680 is a fine card. I've been debating selling my cards to finance one; and then getting a second later on down the road. 2GB on those cards is more than enough at 2560x1600 due to Nvidia's much improved memory handling on the 600-series.
So, I figure the end result looks like this: Keep the GTX570 for now, and upgrade CPU/motherboard and RAM if necessary. A 2500k properly overclocked will be as fast as anything else out there for gaming. When it comes time to upgrade the monitor, then grab something that can handle the resolution properly!