They recommend 400+W PSU's because that's a very safe amount based on how much power your whole system draws and allowing for crappy power supplies.
Basically, an Athlon64 processor (single core) draws around 40W peak at full load. an X2 draws 80-90W peak. The rest of the system (minus the video card) can take anywhere from 40-100W+ (Mobo 20-40W, Hard Drives 5W+ each, fans 2-4W, Optical drives...). Then comes the GPU. The X1900XT/XTX/X1950 draw something like 100W full load. The X1900GT draws about 90W at load. (These are all ballpark figures of course)
You're looking at 250-300W+ peak power draw. With dual GPU (X1900 crossfire or 7900GTX SLI), add another 100W. Many low-quality power supplies over-rate their power supplies (for Generic PSU's I'd subtract 100W off whatever they rate it at, at least).
If you have a 350W good brand (Enermax, OCZ, etc), I'd say you're hedging the line for a single or dual core and an X1900. I'd strongly recommend a 400+W Enermax/Antec/OCZ/etc. Fortron and Seasonic have a good reputation too (get over 400W).
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So it's not that these components are consuming 400W at full load. But the combination of inefficency and peak power draw means that if you have something like a 300W PSU, your system is going to be unstable as hell. Because today's CPU's and GPU's are so power hungry, I'd recommend 400W PSU at minimum, 450W to be safe.
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With that said, I'm using an Enermax 420W Noisetaker PSU (good quality, dual +12V rails) and I'm running an Opteron 165 (dual core) at 2.4 GHz with extra voltage (opterons draw even more power than X2's due to the added cache) and an X1900XTX without any problems. As long as you pick a good brand, 400W and up will be plenty.
Discussion Zen 5 Speculation (EPYC Turin and Strix Point/Granite Ridge - Ryzen 9000)
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