Believe me, I think ANYONE who spends more than $400, just to play a game, is an outlandish moron. (With apologies/condolences to those who bought PS3's)
Right now, the max I would spend on a "Gaming Machine" would be the price of an Xbox 360.
If I owned a Mac Pro, it would be for work. Pwning n00bs in Counterstrike, just doesn't pay the bills.
Not even close.
Now, now, no need to insult us gamers. While I wouldnt buy a Mac Pro to play games my comp is more then $400. Course I dont use it for games only.

Anyways - you dont realise how important gamers are. We are what pushes the computer R&D so much.
We buy games - games are the most hardware intense software out there - R&D makes a stronger comp to make us happy - game designers make new games that are even more intense.
And if I had a Mac Pro it would be for work ... with bootleg running XP from a second hard so I can play some games and frag some fools after a hard days work.

Imagining your iritating boss/coworker/whoever as that enemy unit and blowing it away with a bazooka does save on shrink bills.

I'll admit that the games industry is a multi billion dollar market, and that gamers push R&D, but it is primarily GPU R&D. All the other technologies games utilise are pioneered elsewhere. Think VOIP/VON, or Network Play. Technologies which games appropriated, that definitely weren't part of gaming when they were conceived.
Most R&D I'd say is pushed by the industry itself - all the seriously heavy duty tasks, like huge servers, (database/web/video/audio/client communications etc) rendering massive videos or doing post production work for professional studios, compiling millions of lines of code in massive application development projects, or running massive, mission critical distributed applications, like citywide enabling services, nuclear reactors, air traffic control, traffic lights, etc.
Those things require monstrous computing power, and they're the things that drive the majority of R&D. I'm not denying that games play a big part, but really only in technology that actually appears at the consumer level. Most tech we see at a consumer level today was once professional grade technology, or is a scaled down version of it.
I don't think a lot of people who spend $10,000 on a gaming rig plan to
just use it for gaming, either. If they do, its a complete waste.