Personally, I'd been using an old 230 W power supply that I brought 5/6 years back, and had upgraded the whole system (M/B, RAM, CPU, etc.) to more modern models. I had had absolutely no problem with it since I had always tried to avoid connecting additional hardware to the same power supply. For example, I had externalized one of my HDDs that were the latest model, had connected monitor directly power outlet instead of the computer's power supply, ...
Of course, I'm no longer using such old power supply nowadays, as more new hardware like CD-R/W drives, big HDDs, USB devices, etc. all are draining power aggressively from the power supply.
In conclusion, if you manage your power distribution carefully, even a 250 W power supply is fine for modern computers (unless you are "heavy" hardware installed), though you might have to externalize some of the power drainers to power outlets.
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