There is a thread here somewhere that talks about the missing -5v line on modern ATX power supplies and how it affects certain sound cards. I don't know if that is a huge concern for any other expansion cards though, but if you can find one with -5v, it probably wouldn't hurt.

A 486 PC should need very little power, 15A on the +5V line as you said should be fine. Back in the day most people I know used those cheap generic AT power supplies that came with the cases, and they were generally rated between 15-20A on +5V, but I doubt those things could really provide that much power. So a decent modern ATX power supply that provides the power as stated on its label should be more than enough.

The weaker +5V line on modern power supplies only starts causing real trouble when you get into hungry Athlon territory (at least in my experience).

EDIT: regarding the -5V line, there are ATX --> AT adapters sold on ebay that generate -5V if you don't have it. Also, some of them come with a dummy load on the +3.3V line to "keep voltages stable", but I don't really know if that is really needed, as my knowledge of power supplies is quite basic.

Thanks. I'm measuring <40W at the socket on my 486, so I'm not concerned about total power needed. I just wanted to make sure I wasn't overlooking any differences in how ATX delegates power vs an AT PSU.