msi TwinFrozer 3 Power Edition here... After messing around with my card a bit, the most comfortable overclock I've settled on is 900/1800/1950 +75mV.
The card isn't stable at anything less than +75mV with these speeds, which is pretty reasonable I think. I can probably push it a bit harder if I add more voltage (there's room enough on the slider), but on default fan settings it currently tops at 78-79°C on 99% load (79°C is the absolute max registered on about 5 hours of continuous gaming... 35° Idle and average around 62°C most of the time). I tested more aggressive fan profiles but I'm honestly not too happy with the extra noise... Keeping the same curve, but pushing it up so that it always spins 5% more than the default manages to cut off a good 4°C, but it gets really noisy really fast. Basically I'm leaving it at that because I know I can't enjoy playing next to something that sounds like a jet engine just to manage the extra heat a few measly Mhz would generate
If you're more tolerant towards fan noise, or if you're adventurous and don't mind daring it at higher temps (or if you're a cooling virtuoso and find a better cooling solution), you can most likely find stable settings higher than 900/1800/1950 on pretty much any card. I don't think I've seen a single poster in this thread who couldn't manage a stable, acceptably cool 900/1800/1950 setup, that seems like this card's de-facto safe-but-powerful near-guaranteed easy-to-manage overclock. Like many have said though, they're not all built equal, some cards can go higher and still stay cool, others will tend to heat up more easily, others need more voltage to stay stable, but anyone should be able to reach that.
Regarding voltage and stability, thank god Windows 7 doesn't blue screen on you when the graphics drivers crash like older OSes would. Nowadays it's more like "Oh it crashed. *CLICK* *CLICK* back in business"
Regarding 3DMark11 scores, I hate that although my score is above the majority of other people with comparable machines, 3DMark11 insists my score is low because this *one* guy is hitting über high scores with a nitrogen pump or something cooling both his CPU and GPU at extreme overclocks. That's hardly the average setup and hardly what I'd compare mine to.