White boxes in games would mean you're unstable. Use the evga tool, and a program called furmark to test stability. Certainly dont use rivatuner and evga precision, one or the other. I prefer the evga tool.
Increase the speeds a little bit, run furmark for half an hour or so and look for signs of misbehaviour. If temps go over 100 stop this and sort out case airflow first.
Once you've got speeds you're happy with, and any more cause graphics to mess up, you can run the folding@home application offline to test it further. Furmark will see if it messes up in games, folding will determine if it remains mathematically accurate. Folding is a more difficult test to pass, and will mean lower speeds to remain stable. Many people skip this step.
The last step, which most people don't seem to do either, is to make the overclock permanent. So if you remove the tools, reinstall windows, move the card to a different computer etc it remains overclocked. This features dos, a floppy drive and a program called nvflash and another called nibitor. It's very simple, but for much of it you'll by typing into a black and white screen and this puts people off. Along with knowing that it is very, very unforgiving of typos. If you want to go down this route once you have stable clocks in software, just ask and I'll offer a much more detailed response.
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