First plan - Get a decent power supply. Don't you dare doing anything before that.
Now, if you don't want to replace your entire system, the best upgrade for you is a graphics card. As nekulturny said, you better off getting a GTX 670 than a 680, and Corsair's TX line of power supplies is a great choice.
For the cpu - You can get an aftermarket cooling, which you can also use on different cpus should you decide to upgrade, and overclock the hell outta this cpu for now. Then you should see if it's enough, and if not you should consider getting a 4-core sandy bridge or (preferably) ivy bridge.