550 is a pretty tight budget, but something like this would do:
http://ca.pcpartpicker.com/p/DKQCnQ
I left it at that price since I usually don't like power supplies that come with cases so the last 50 bucks is to find a better unit there.
I assume you don't mean 550 including a monitor, keyboard, mouse... etc.
If you want to just keep that GPU and want a focus more on the rendering then something like this might make a little more sense, depending on the applications your are using:
http://ca.pcpartpicker.com/p/zhNkK8
No room for the PSU replacement here, but your power usage will be pretty low anyway so depending on how much you care about the rendering speed you can either sink the 50 bucks in to the first machines CPU to...