Id say leave it at 1.5v unless u run into problems. I tried raising mine to 1.6 and it causes what seemed to be throttling on my 6800gt, computer locking up for a few seconds while doing anything 3d. my temps never went over 65c either, perhaps others will offer more insight, but in my experience 1.5 is fine.
You can raise the AGP voltage to try to fix an unstable AGP port. Sometimes raising the voltage will stop data corruption. If you're not having any problems with your video card then it's best to just leave it at the standard value of 1.5 volts.
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