The most important thing is the amperage(A) on the power supply 12V output. It'll be on the label of the power supply. It'll say something like "12V output 18A". If it's 16A or less, I'd say no, don't do it. You're likely to need at minimum 14A on the 12V just for the card and CPU. That's with the PSU operating at full load, which is not a good idea. Ideally you'd want more than 18A on that output. That gives you some headroom and keeps the PSU from working so hard.