I typically use a modern Seasonic 20 amp PSU and have no problems even with Palomino 2000 + GeForce 6800 ultra on KT333 without ATX12V. I actually have 3 different ones at the moment, from 300W to 550W. Even the 300W doesnt flinch.

I am not sure I would suggest buying old, used PSUs. You may need to refurbish them.

What decides the current requirement for 5v?

For instance my A7V133 manual only says,

IMPORTANT: Make sure that your ATX power supply can supply at least 10mA on the +5-volt standby lead (+5VSB). You may experience […]
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IMPORTANT: Make sure that your ATX power supply can supply at least 10mA
on the +5-volt standby lead (+5VSB). You may experience difficulty in power-
ing ON your system if your power supply cannot support the load. For Wake-
On-LAN support, your ATX power supply must supply at least 720mA +5VSB.

5V rail is used for PCI bus power, disks and for CPU voltage regulators (but if mainboard has the ATX12V connector then 12V line powers up the CPU).
I don't know how accurate or correct are my calculations, but I usually check TDP of a processor and treat it like power consumption; Duron Spitfire has TDP of 25W and needs 1,5 volts, which is generated from the 5V line by voltage regulator. If we assume it has 90% efficiency, the actual power consumption would be about 28W and if we divide it by 5 volts we get 5,6 amps of current in 5V line just for the CPU. A single PCI card should never use more than 25W so it would be 5 amps max. and that would be used by 3D graphic cars. Soundcards, network cards and other stuff probably will not use more than 1,5amp for single card (if it doesn't have heatsink it can't use too much power :p). And each hard disk or CD drive uses about 1 amp. So Spitfire Duron with PCI Voodoo3 or 4, CD, HDD, network and sound card would use about 16 amps on +5v. With most power-hungry Morgan Duron 1300 that would be 24 amps.

According to Wikipédia the 950 Spitfire Duron has a 41.5W TDP, not 25W, so as per your calculations it sould need a bit less than 20A. Do you guys have any experience with that Enhance brand? Or would I be better of getting a 20A PSU from the likes of Seasonic or Cooler Master?

Spitfire datasheet

Thanks, adalbert. That's only missing AGP, which I found a maximum for over here.
https://en.wikipedia.org/wiki/Accelerated_Gra … wer_consumption

I use a 350w powermax to drive a barton 2800 on a kt333 board

Well, I just found out that I have a spare PSU rated for 35A on the 5V line. The 3.3 isn't as beefy but I guess it is fine. Also, it is an MTek, which isn't exactly Seasonic or anything like that. It seems to be a perfect fit for that system, though, as it is has a 20P ATX connector (no extra 4 pins, just like the motherboard). I'm just curious about an extra connector there - it looks like one of the halves of an AT power connector, but it's got an orange wire, so it has nothing to do with AT. Could it be an extra conector for high end motherboards before the extra 4 pins were standardized? Also, it has one bulging in what appears to be the 3.3V output. After I set the system up I'll take the PSU apart completly and see if it is worth recapping.

This one?
https://en.wikipedia.org/wiki/ATX#Original_ATX

A supplemental 6-pin AUX connector providing additional 3.3 V and 5 V supplies to the motherboard, if needed. This was used to power the CPU in motherboards with CPU voltage regulator modules which required 3.3 volt and/or 5 volt rails and could not get enough power through the regular 20-pin header.

Yup, that's the one, thanks.