I only have a laptop, but it is strong enough for TBS up to now. It's lagging with Civ4, but still capable. So I don't bother... yet. My interests grasp more of certain things out of games, and I DO buy computers for other reasons
I know I game I wanted to see the demo: my laptop would not take it so just too bad. Sad, but first things first.
You see, some gamers get jaded or interested by other things... and they only keep up with their favorites, maybe as fervently as before but just with less time to spend. And some gamers just don't have parents buying them a computer every 2 years (some also have slow computers, but good Internet cafes where they became avid "gamers").
my cutie (bought second-hand, 6 months ago):
ThinkPadR50
Centrino 1.6
Radeon 32MB DDR
512 DDRAM
30Gig
See? No big beast, but it does what it's asked and does it faster than my previous desktop (700MHz, TNT2Ultra, 256SDRAM). Priorities change, even if I am still interested in games as games and works of art (with its industry). I'm anything but a classic case though, being ex-centric among ex-centrics, but it might be more common in TBS communities.