lost power unexpectedly

Windows 8.1 Pro 64bit
Case STYLISH SILVER/BLACK TRIDENT CASE + 2 FRONT USB
Processor (CPU) AMD PHENOM II X4 965 SOCKET AM3 (3.40GHz/8MB CACHE) - BLACK
EDITION
Motherboard ASUS® M4A785TD-V EVO: DUAL DDR3, S-ATA II, 2 x PCIe x16, 1 x PCIe
x1, 3 x PCI
Memory (RAM) 8GB CORSAIR XMS3 DUAL-DDR3 1333MHz
Graphics Card AMD Radeon R7 200 Series
PSU 450W Quiet Dual Rail + 120mm Case Fan

Hi Folks I hope you can help.
I have just upgraded my graphics card, now when I try to convert mp4 to avi my system just shuts down. I think the problem is my PSU is not powerful enough (this is where you guys can help) When I check my event viewer I get the error,
source Kernel-Power
ID 41
lost power unexpectedly.

So what do you guys think? If it is my PSU can you recommend a new PSU what would be compatible with my system.

Thanks
H