CPUs aren't 5V or 12V, motherboards are that's why googling the CPU won't find much. The CPUs themselves just get their much lower voltages (1.4-1.5V core, 1.25V bus) from the motherboard.
So370 (or indeed Slot 1 ones) motherboards almost all run the CPU off the 5V line (only some rare industrial boards do 12V). Also AGP gets most power from 3.3V. So you need a PSU that can handle relatively high currents on those lines.
As for being better off with old or new PSU, that's subject of a lot of debate. I personally lean more towards good-quality old PSUs if available (~2000-era ATX is plentiful for almost no money where I live), but an efficient new PSU that can handle the 5V requirements is fine too. If you go for an old one, check the caps before use. Either way, be sure to go for a quality PSU, as - old or new - crappy lightweight ones can't deliver what they promise and risk your components.