280x's get to 300w in a hurry.
CX430 = $40.
280x = $400.
Not worth the risk if you blow something up.
280x get to 300w? What card are you running that gets to 300w? As I said, mine started at 270w, and after undervolting, dropped down to 230w per GPU.
XFX R9 280x's at stock volts / clocks because mine came locked.
Undervolting is another matter. I did not assume the OP would be doing this.
However, to recommend a card that must be undervolted to maintain a safe load on the PSU is reckless.