My personal spin on what is happening with the (lack of) stampeding PC development is.... Microsoft.
Their insistence that you upgrade your OS to get next generation DirectX, combined with Vista being wholly unpalatable, combined to keep a large segment of gamers 'last generation'. That combined with the recession makes it rather risky and pointless to make games for insane rigs.
We're starting to see higher requirements again, though. If my theory is right, that may be due to Win 7 having gotten the consumers' 'stamp of approval' in a way Vista never did, and will accelerate further.
My current gaming rig is a $1000, four year old box, and it still works just fine. Well... almost. The hard drive is going, and I'm thinking of getting a new rig (It started chugging a bit with Mafia II). Friend of mine bought a new PC recently...went totally overboard, spent roughly 5000$ on the components and he's not even a gamer. (Although he plays the occassional game). He just wanted to see how far he could go, I think. Me, I'll get another $1000 rig. Good screen is important, though, unfortuantely mine's still working perfectly well so I can't honestly go upgrade that.
(Samsung 226BW, excellent screen but out of production now)