GTX460 768MB sucks.
Get a real one with 1GB.
PSU is marginal but should be okay if it includes PCI-E attachment.
A GTX460 1GB can require up to ~24A apparently,
Where did you get that idea from?
It seems that even a heavily OC'd and overvolted card barely uses 18 amps.
A more standard one will struggle to use more than 11-12 amps.