I've got a compaq with a 250W power supply and I'd like to upgrade it. I've built many machines before and I'm very comfortable with hardware. However I've never replaced a power supply before so I'm not sure how to tell if a new one will "fit" the old one. Are all mb power connectors standard? And are all PS physical sizes ( not wattage) standard?
Motherboard power connectors follow ATX or EPS standards, and generally speaking all modern PSUs have the same connections. The physical sizes for ATX PSUs are standard, I'm not fully sure whether a regular sized ATX PSU will fit into a µATX case.
Old OEM systems from Compaq, Dell and etc had special PSU's (special connectors) Newer OEM systems have std connectors it is more a matter what form factor the PC is. For example a mini tower accepts an ATX size PSU. The physical size varies especially on the length since the mounting is std for ATX.