Mostly gaming cards of the mainstream or highend variant. We build 10% office / 30% multimedia / 60% gaming PCs, some of which are very expensive. I'd say that we currently mostly use 700-series cards. No ATI, though.
The most used cards should be 780 and 770.
I'm in switzerland, and switzerland seems to lack dedicated gaming PC manufacturers, so that is what we do (for another company, though). The office PC market is quite saturated with the big brands, but gaming PCs that are actually good for gaming and offer (somewhat) good value are rare.