Back to OS X, Firefox 3.6 EOL
Well after all my big words I couldn't do it - I'm now back on OS X.
Why? Well there wasn't really one reason, rather a bunch of little things (the trackpad under Linux was a bit fussy which made using the machine a bit annoying, no sleep/resume, the machine ran a bit hot, couldn't play StarCraft/Diablo III/Doom 3/Aliens vs Predator...), basically this old machine simply runs better under OS X.
With Safari out of support for over 6 months now and Firefox 3.6 approaching End Of Life I've switched to TenFourFox, a PowerPC version of the latest long term support version of Firefox (version 10). I hadn't switched earlier as no plugins worked, or so I thought - actually all I had to do was enter "about:config" in the address bar, search for "tenfourfox.plugins.enabled" and double click the line to set it "true". voilà, Flash, Quicktime and Java now all work (though definitely install Flashblock as the last version of Flash for PowerPC has known vulnerabilities so you're better off being able to choose when to run it).
I'll eke a bit more life out of this old box yet!