You want the game running as fast as possible, rendering as fast as possible, processing as fast as possible, using as much memory as is available, eating as much video memory as the card has. Simple: You want to max out the resources available to your game. And when I say max out I mean 100% completely max them out. Games like Oblivion, Doom 3, Microsoft Flight Simulator X, Spore, Sims2, and tons of others need every ounce of every resource the computer has to offer. If the computer has 3 gig, use it, if it has 6 gig, use it. Of course there is a buffer zone so the system doesn't run out of memory but most games will attempt to use everything they can.
What is absurd is that they eat 100% CPU.
No, we mostly don't care about the background processes, but we DO care about the electric bill and there's where the problem lies.
They don't have to eat 100% CPU to immense the player in a wonderful experience.
Use as much as they need, and no more.