yeah, the amps on the 12v rail is very important.
It is also the hallmark of a good psu, more so than the "wattage"
a 400 - 450 Watt powersupply with 18a or greater on the 12V rail is recommended.
We are using such on a rig with 6600GT, Athlon 64 3000+ and 1gb RAM.
No issues.
Anything less and you could be in trouble.
Symptoms will range from random lockups to the screen blacking out.