The Industry Demographic
The bottom line is fairly straightforward: know your market. Are you making a game for yourself or as a product? The best combo is a title you really enjoy making that you hope lots of other people will enjoy playing (and pay you to do so). Decide on the game you want to do, research the demographics of the likely audience for it and plan accordingly.
Yeah that's great and all. Someone out there know where there is public information on user and hardware demographics of gamers? Of everyone out there playing the Quake/Unreal/Half-Life series games what do you think the average player is really running? Seems to me that most publishers and publications would closely guard that information as they collect it.