Well, it really depends on what the motherboard manufacturer requirements are. They usually take into account the other hardware. I for example have the UT NF4 SLI Expert board from DFI. They require a 500 watt PSU minimum to run it.
The more horsepower you put in the machine, the better. Too little will definately hurt you and/or cause the machine not to run.
I have an Antec TP2.0 550 Watt Unit in my machine. It's a good PSU (some will say it's not) and does the job nicely.