Forgive me if I am incorrect, but are there any 64-bit games that require a 64-bit OS?
I am on 64-bit Win7 so I would not know. Mainly just interest, my assumption was that only certain programs needed 64-bit but not currently games.
As for your question, exactly as SR-71 Blackbird states.
i don't think any game at the moment really utilizes 64 bit only(at the most, utilize ram so that background tasks can be ran while gaming as well), but the first group to probably move in that direction is DICE, as they have plans on making their games 64 bit soon.
games in general are 32 bit because it can be run on most systems, and more potential buyers mean better profits.