Base Clock: Minimum clock the GPU will run at
Boost Clock: Clock speed that GPU will boost to in demanding scenarios.
Usually when you overclock, you must be very careful of adjusting settings. I recommend using PrecisionX or MSI Afterburner for overclocking.
PrecisionX: http://www.evga.com/precision/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+EvgaNewsRssFeed+(EVGA+News+RSS+Feed)
MSI Afterburner:
http://event.msi.com/vga/afterburner/download.htm
These tools will allow you to change the base clock speed, memory clock speed, power limit, etc.
So, let's walk through what;s going to happen.
1) The most important part of GPU overclocking (and most beneficial TO overclock) is the core clock. That will determine the speed at which your GPU operates. So, what is a good idea to do is add 10 MHZ to the core (or base) clock speed and then stress test. FurMark is a good stress tester. After about 10 Minutes of stress testing you can move on.
2) Keep adding MHZ in increments of 10 (stress testing every time you do so) until the card starts crashing or having issues.
*KEEP IN MIND. WATCH THE TEMPERATURES. MAKE SURE THEY DONT GET TOO HIGH!*
3) Once you have reached a point where the GPU starts crashing, back off the clock speed by about 10-15mhz and then stress test for about 1-2 hours to ensure stability. If all goes well, it is a stable overclock!
Once again, I recommend MSI Afterburner for overclocking as that is what I use.