EVGA has one of the best warranties around. Overclock, go for it. Change the cooler? sure just keep the old one and put it back if you have to rma. Very few companies will offer a better warranty then that.
The GTS 640 also overclocks well. Just clock the Core and Shaders separately(RivaTuner) other then that you will hit a wall sooner. When you hit a max on the core, back it off a bit to be safe and then start with the shaders, then last the mem. Also note the clocks on that card go in jumps. so 575 may actually be 585.http://downloads.guru3d.com/download.php?det=163
You can use ATItool to test the clocks(very few things heat a card like the scan for artifacts button)http://www.techpowerup.com/atitool/