Here are my thoughts:
1. The requested ratings for power supplies were so high back then because 95% of power supplies being sold were absolute trash. If you read some of the Johnny Guru and Hardocp power supply reviews back then, you would have seen some that they actually bothered to review that wouldn't even meet 50% of the rated output before they fried or caught on fire or blew up. And those generally weren't even the cheap crap that was the norm back then.
2. I have KT7A setup with a Barton Mobile 2800+ that has a newer Seasonic power supply in it. Definitely not one of the higher 5v and 3.3v rail power supplies. It overclocks fine to about 2.3Ghz. I was thinking that the motherboard was the limiting factor but now I am not 100% sure as I forgot about the power output ratings of the power supplies back then.
3. Because of this being brought up, I ended up buying a couple PSUs off of eBay just now. a NIB Antec TruePower 550w (first version) and a used ToPower 650w. Both were not expensive and I did a lot of research on brands from back then.
4. Here are some brands to look for in no particular order:
1. Antec
2. ToPower / ePower
3. Zalman
4. Enermax
5. Seasonic
6. Zippy
7. Super Flower
8. Silverstone
9. RaidMax
Here are some new (open box) Zalman 400w PSUs for $29.95 shipped in the USA that meet the criteria you are looking for:
https://www.ebay.com/itm/ZALMAN-ZM400B-APS-40 … ly/264662939178 (this is the newer model)
https://www.ebay.com/itm/ZALMAN-ZM400A-APF-40 … ly/142787727379
https://www.ebay.com/itm/ZALMAN-ZM400A-APF-40 … ly/264634617097
used ePower 650w for about $45 shipped:
https://www.ebay.com/itm/ePower-silent-engine … 50/233443925702
used Antec TruePower 550w for $33 shipped:
https://www.ebay.com/itm/Antec-TruePower-550- … ly/173831986643
There are some new power supplies with decent 5v and 3.3v rails for pretty cheap.. as in $24.95 shipped, but I am not convinced that they aren't crap so I am not going to even bother linking.. just search for 650w power supply and search by price lowest to highest.