Ta Hugh - Ive been thinking of having a similar set up and using linux mainly for going online. Ive recently have been trying antiX which apparently (distrowatch) has a connection with MX.My iMac 7,1 (20" C2D 2.4 from mid-2007, 6GB RAM and 256GB SSD) refused to boot any version of Mint + other distros except MX-21 Wildflower 32 bit XFCE, which I now have dual booted with Snow Leopard.
So I now have my favourite Mac OS complete with support for PPC apps and games, plus a modern secure OS for internet use.
Cheers
Hugh
Found antix was quite good as theres a preboot menu that allows me to 'Xorg=safe' to fix the screen prob I usually have with linux.
One prob I discovered with antiX is lack of fan temp control while watching a movie online: Sudden appearance of vertical bars across the screen due to GPU over heating. Ive since added a temp reading at the top of the screen...as suggested in one of the antiX discussions.
Ive only been using it from a USB stick so far..