Anything close to a trigger was Crysis for me. Had a Pentium D 820 with a X1950XTX and the game didn't run well even on medium. Other games ran fine though... I was moving to a new apartment, so I bought a new PC as well. Q9450, HD4870 - this thing plowed through everything (Crysis still ran like sh!t though, this time at max, heh...).
In general no single game was pushing me to upgrade. When I feel games don't run well enough for me, I just do the upgrade. My OC'ed HD5850 runs everything I play like a champ, so I can't really say when I'll upgrade now. I need to see how Skyrim runs and then decide. But I expect the game to be smooth...
Before the Pentium D I had a laptop with a Mobility 9800 (was a 9600Pro-level card). Had a Pentium M 1.4 and a slow 4200 RPM (bought it when moving places too

). I had to run so many games on low settings... Then I bought the Pentium D with a 7800GT and had so many problems with the GeForce, this triggered a switch to X1950XTX.
Another example... moved to a new apartment during university time and got a PII 233 with a TNT2 card.
So there
