Well overclocking has gotten in much easier. The ASUS P8Z77-V includes a utility that does overclocking for you automagically. And with parts like the Intel iCore 5 3570K, it allows overclocking and even warranties for it. We got to 34% overclock (3.8GHz to 4.4GHz) at the CPU this way and the GPUs got from a stock 980MHz to 1050MHz with this trick.
Using the tricks from this, we nearly doubled the frame rates. So the first cut of the Heaven Benchmark was 26fps, but these tweaks, we got to 44fps. And that is at 4k x 1024 pixels (eg three monitors worth of pixels) with everything turned on from 16x anistrophic, high shaders, texturs, trilinear filtering and 8x anti-aliasing. Pretty remarkable what a pair of GTX 670s can do!
But if you want to get a little better, there are geek guids. The Overclock.net seems pretty good. It let's you do a quick thing based on the the eVGA tool and CPU-z using the Heavenmark benchmark. You basically run the fans at full speed, set the Power overtarget to 112% (means 112% of the stock wattage), the voltage to the GPUs to max and then go up in increments of 20MHz until it crashes, then down 5MHz.
This definitely has an impact. The first Heaven benchmark I ran at "stock overclock" if that is a term was 26fps, by increasing the GPU by 61MHz to 1.12GHz from 980Mhz and the memory clock on the card by 100MHz from 1.5GHz.
All of the Kepler-based GPUs (670, 680, and 690's) are a very unique breed of GPU. Gone are the days of manually increasing voltage to stabilize an otherwise unstable overclock. Now, the user must use a great deal of finesse, and a ton of trial and error, to maximize the potential of their overclock. We now have to worry about dynamic clocking, dynamic volt changes, temperatures, and power draw in-order to reach a maximum stable overclock.
via ~~The GTX 670 Overclocking Master-Guide~~.