What mac to use with my Pro Tools set up

Hey guys, just wondering if you can help me out. I have a G5 power mac, Pro tools M powered 8 and a Project mix io. I've been having some problems with my mac and I'm now looking to upgrade it. I know you can spend thousands upon thousands of pounds on a new mac but i haven't got those kinds of funds so was wondering what mac would suit me. I'm a college studying Sound Production and will be using my set up for Sound Design, music producing, dubbing etc. I was looking at Imacs but not sure if this is what i'm needing.

Any help would be appreciated

Thanks