My dad is a graphic designer who got his start doing desktop publishing on a Macintosh II using QuarkXPress. He's faithfully stuck with the Mac since the '80s and currently has a trash can Mac Pro, with which he is mostly happy. He's never billed anywhere near $500/hour as most of his work is for small print periodicals (<50k circulation) and online presence for small nonprofits.

Currently, his only outstanding need is for better render times on Final Cut Pro (rendering videos of conference proceedings for YouTube), but I know he'll balk at $6000 for the new Mac Pro. I don't know what I'll recommend.