18A on 12V rail 550Ti??

I'm guessing that if you're just underpowering it the gfx card would not run as good as it would with the proper amperage

So technically it could be fine??

with 3d running the machine could become unstable

Other than a high chance the power supply will just plain out fail and give out some cancerous smoke alongside a good firework show and a pc that has everything fried. Not much.

it depends, it could fail in such a way as to damage other components

Right I won't risk it so

i'm trying to help somone at the moment who was running a system off a 250W PSU that was at least 50W short in terms of 12V power, currently nothing works except fans, i'm loaning him a PSU and crossing fingers for him....

More than likely the PC will crash when you put the Vid card under load. Some power sensing Vid cards may not boot at all with such low amps. available on the 12v rail.