I think the worries about the 5V line are overrated. Take a look at how much power a retro system draws from the wall.
As an example, I tested an old Dell from 2001, having P4 1.6 Willamette + 1GB Rambus mem + GF3 + SB Live + HDD, and the whole system drew about 131 watts from the outlet, under load. If you multiply that by a very optimistic 80% PSU efficiency, that's about 105W total system consumption on the DC output side.
Now if you replace that with a P3 and SDRAM, then you can subtract another 20-30 watts from that figure. So unless you plan on running a top of the line Athlon Thunderbird, then 120W combined 5V + 3.3V output should be fine.
Another aspect of power supplies to consider is how the output is regulated. More expensive units have a DC converter that derives 5V and 3.3V outputs from the 12V line. These units are less affected by crossloads that place a disproportionately high load on the 3.3v or 5v lines.
Cheaper units are group regulated, where having too much load on 3.3v or 5v lines would cause the voltage on the 12v line to go higher than normal.
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