Q6600 @ 3Ghz
XFX 6870 2GB
and so on.
I'm being heavily CPU limited in games like Chivalry, SotS 2, Mechwarrior: Living Legends, among other things.
$400 is quite a bit to spend on an upgrade, which is why I'm hesitating. There'd be nowhere to upgrade from a 3570k, really, and even for Haswell it appears to be the same (rumors being that Broadwell is only going to ship soldered onto MBs). And on the Haswell note, it doesn't appear to be anything revolutionary for desktops, since its biggest feature is significantly reduced power consumption.
If I were to upgrade to a 3570k, I'd be buying a 212 EVO cooler and OC'ing it out of the box. I'm not interested in selling my C2Q, as I have other uses for it.
If you believe you are heavily bottlenecked, then I think you should upgrade to Ivy Bridge now. Haswell is expected next year between April & June. So you are looking at waiting at least 4 months.
I don't think performance will improve much because I believe Intel in focusing more on power power consumption than performance. Therefore, I would say that performance increase would by less than 10% assuming the same clockspeed.