There is nothing wrong with that 12V rail.
If you buy a second PSU your going to have to find somewhere to fit it, mod them or buy an adapter so they start up at the same time, and it's not really going to make much difference.
Look at it this way - you've bought a mid range card, and your PSU can easily handle it, but your tricking yourself into thinking you've got an amazing PC and the PSU can't cope.
It can.