What's your psu now? Power supplies can definitely affect stability and how far you can oc. However, it's still luck of the draw. Your chip might be at it's limit already.
I have a 430 TruePower and love it. Quiet, powerful, easy to install. But set your expectations first. What do you do if you can't get a higher oc with the psu? Return it? I would suggest that if you upgrade your psu, don't EXPECT to oc higher. Other things will benefit from the psu upgrade. Otherwise, save the $70 for now and maybe get another chip or some other goodies for your rig.