Power Supply Calculator
I accidentally stumbled across this site that has a fancy calculator that helps you calculate the required power supply for your system.
Personally I would always add 50W to the result that this calculator gives just for the sake of safety and expansion (you might want to add a DVD drive or extra HDD to your system - its always better to have room to grow than not enough.
It does seem to indicate that I will have room for lots of expansion with my Antec TruPower 480W PSU.
M