What size PSU will I need

My brother just got a new pc going

Asus M2N-MX SE mobo
AMD am2 4200+
2 gig Mushkin DDR2 800mhz ram (2 x 1gb sticks)
Sapphire HD2400pro graphics card
1 x IDE dvd Burner
1 x IDE hdd

How big of a power supply would I need for that..
I had a 400w in there but it kept cutting out
So I pulled out my 550w psu from my old pc and put it in
and it works fine...

But I just am just wondering what size PSU Could run that PC so I can have my old PSU back....

Thanxx in Advance..

 

a quality 450watt power supply will do it just fine.Just look at the 12v rail and make sure it draws at least 25amps or higher

 

Ok kool Thanx Heaps for the reply...