Confused about power supplies for motherboards? Me too. This will help:

http://www.theinquirer.net/en/inquirer/news/2003/08/30/power-supplies-become-increasingly-expensive

ATX = 20 pin plug, your Pentium III’s ATX connector and PS
ATX12V = 20 pin atx connector + 4-pin plug for “Additional 12V”
SSI = Intel spec, which, among other things, defines the “EPS” “enhanced” ATX specification
EPS12V= Power supply with 24-pin EPS12V connector, plus one 8-pin “additional 12v connector”.