Start off pretty slow (10-20mhz) bumps for both the GPU clock and the memory clock. Keep increasing until the temps become too much, or the card starts to artifact / misbehave.
ATI overdrive doesn't adjust voltages so there will be a wall it will hit with 'stock' voltages.
For stress testing, you can either use the tried and true Furmark
or the newer kombuster that is part of MSI AfterBurner
*NB You have to install install kombuster separately to MSI AB.
Leave adjusting the voltages alone for now and find the 'wall' with stock volts first. And yes, it can make a difference depending on the amount of overclock.
Watch your temps when stressing the card - don't run them for too long either (5-10mins) as they will place a very heavy load on your card. Stop the tests immediately if you notice the temps are too high, or the card starts to artifact.
(Artifacts are little graphical glitches/dots)