With a good but underpowered PSU, you will have seemingly random reset/reboot cycles if you are lucky. If you are unlucky, you will ruin your PSU. If you are really unlucky, the PSU will take other parts (motherboard, CPU, GPU, and/or RAM) with it when it fails.
Thanks for the replies! I think I'll first find out how much power my PC is using now. I do remember it using around 250W max, running a high end game, on my old GPU. Does anyone have an idea how much a GTX 560 TI uses on power on average?
As mentioned I have seen a whole variety of cases with underpowered or low quality PSU's.
Not even booting up, ruining whole setups (low quality), to constant reboots and just general performance issues. I've seen enough to know it's usually not worth the risk of just "trying it out". It's better to check with a reputable PSU calculator, or check with some professionals or experts.
Tried the calculator, I don't even know have the stuff they're asking me. =P
Here's the rough specs from my PC:
Motherboard: Asus Crosshair III
Processor: BE AMD Phenom II x4 965 (revised, 125W (I think))
Two Sata Hard Drives
Graphic Cards: Gigabyte GTX 560 TI (and the older, optional AMD 5770)
PSU: 450W Corsair
I'm guessing that I will need a better PSU if I want to hook up that extra GPU.
If necessary, I can add better details later from Speccy, or some other System