I see. You have slightly different clock speeds than the reference model I linked on the Geforce.com site. You're fine at those clock speeds, since those are the MSI default for that particular card. Afterburner should be saying 810 MHz core and 500 MHz memory, just as you originally stated. I just wanted to make the point that your memory clock speed is 2X the Afterburner value because it's DDR memory; NOT because you have 2GB of memory as was indicated earlier.
So start from your default clock speeds and overclock from there if you wish. Overclock a little, test it out with Kombustor (comes bundled with Afterburner), overclock some more, and repeat until you see anything weird happen (anything at all), Then back down ~10 MHz and leave it at that. You can work the core and memory at the same time, or work each one separately. You should probably begin by expecting ~100 MHz increase maximum on both core and memory, just as a basic frame of reference. It might be much more or much less, but sort of shoot for that as a starting expectation.
Unfortunately, you need to realize that the GT610 is probably the lowest model video card that Nvidia makes and really isn't a gaming card. No amount of overclocking is going to change that, for the most part.