Power Supply 400 Watt Halts my Computer

I was told by Soyo that my motherboard required greater than a 250
Watt power supply. So I got a 400 Watt power supply. I installed it.
It runs fine under no load, but as soon as I put a load (now this is
key) on the graphics card it halts my system. I don't know why that
is. There are many features that I hate that come with it, such as
halt when overheating detected in power supply among others.
However, when the system halts the air from the power supply is quite
cool and I do not have that many peripherals connected to it. My old
250 Watt power supply would never halt. I have a Bayone drive
installed and that is the reason I upgraded to a 400 Watt power supply
(in order to run USB 2.0 and Smart Card reader) Soyo said I needed at
least a 350 Watt. I thought this was crap and still do but..

So if there is anyone who believe thay know the reason for my system
halting please answer.

SPECS:

400 Watt Poer Supply
Sony 48x32xx28x16x DVD/CD-RW
Soy KT333 DRAGON Ultra Platinum Edition
512 MB Kingston DDR PC2700 DDR
Athlon XP 1800+
Bayone (provided by Soyo)
Que Firewire PCI Card
ATI RADEON 9700 All-In-Wonder
IBM 40GB HD
Western Digital 120GB HD

Thanks in advance