My opinion is that what's holding you back is your video card. Here's about the closest comparison I could find to your Q6600 @ 3.4Ghz vs a 2500K: http://www.anandtech.com/bench/Product/ ... 0.49.48.47
In games, they're not too far apart. Though the 2500K destroys the Core 2 Extreme Quad in pretty much everything else.
Compare that to the GTX 660 review with the skyrim as a benchmark: http://techreport.com/review/23527/revi ... ics-card/5
Take notice to the GTX 260. The 260 and 4870 are fairly equivalent on many games, though it looks like Radeons don't do as well as the GeForces on Skyrim. Note that that's on 2560x1440 resolution but lowering the res to 1080p probably won't get you above 60FPS with your 4870.
My advice is to absolutely update the video card. You will notice the biggest increase from that.
If you're really strapped for cash though I would honestly just recommend turning down the resolution and upping the image quality settings. If the upscaling from 720p to 1080p bothers you, see if you can't just tell the graphics driver or your monitor to display the image as 1280x720 on your monitor and leave the other pixels black (though the smaller size of the image and black bars might bother you more).
Here's what I'm talking about by the way: http://support.amd.com/us/kbarticles/Pa ... aling.aspx