I have a second-gen dual-2 G5 (the middle one), 2GB RAM, ATI9600, Medea RTR array, Decklink SP card, previous-gen Cinema HD display on the mac side, and a new Dell XPS Gen2 notebook with 2GB of RAM on the Windows side. Lots of external FW and USB drives help warm up the office. 🙂
Don’t ever get the bottom Mac if they’re offering three. You’ll be limited.
I don’t know about the extra juice of the 2.7. I’d put the money elsewhere, into RAM or apps or a bigger monitor.
My 2 cents,
Steve