So I'm getting a PS4 pretty soon and while I do plan on using my own name I still want to change my PSN name cause it's really stupid. My question is: If I change my email address on my old PS3 account (gmail account) to my secondary email address will I then be able to use my gmail address with a new PSN account?

Yes, you could also just ask playstation support to change it and they will.

@corruptedevil: Oh really? I had just heard that they wouldn't allowe you to change your username?

But I'm gonna try and write them instead. Thanks!