I respect that opinion, but also tire of it. Unless you're cutting Peter Jackson's next epic, an iMac can easily rough cut picture for all purposes, albeit taking a little longer.
I think this whole techno-bigger-better obsession seems to hinge on the fact that many want to hide behind power-systems for lack of resourcefulness and creativity. You can have a dual-quad-quad for FCP 10, but it won't make you a better editor; you can run Motion on a Mactel, but that won't make your effects enhance your story.
All this to say, you need a system within your budget that'll serve your purposes enough to TEACH you something about technique, mechanics, aesthetics. You can hire tech-geeks (i.e. technicians) later to clean up audio, video, effects, titling. But they'll work for you, pay for your system, while you go on your way to making better art.