really my rule of thumb has always been when it's a significant upgrade.

for a long time i didn't really upgrade until it was a 4x increase over my old. certain exceptions were occasionally made. nowadays i'm a bit more opportunistic in my upgrades. but i still seek out 'meaningful' upgrades. upgrades that are a decent jump over the old. typically 50% improvement in performance, or upgrades i can get for really cheap.

It depends on what you need. I think usually you can get the best bang for buck by buying the now previous generation when the new one is released.

Graphics card. Not even once.