It has more to do with being a hard worker and loving what you do, I'd imagine. I'm hoping to start my own PhD in Oct and my concern is not about my intelligence (I've already been accepted) but about remaining focused for the next four years while my friends are earning money in industry!
I work with a PhD student who is quite clearly doing the PhD to become a doctor, he's miserable as sin. I think the only way you can survive a PhD is if you are passionate about the subject, being inquisitive is also essential.
unless they ask u to do an IQ test they will not ask you whta s ur IQ, they will select the right person during he interview. they look at good academic background, your marks plus experience so make sure u have those
IQ measurement are very inaccurate, hovever to answear ur question on a larger scale i would say that you need at least an IQ of 130, I don't think that anybody will check though