Hi Steve (long time!)
I'm still using my set up of several years ago - when I was more active on these forums - it is FCP 6.0.6 and Mac OS 10.5.8. I'll stick with this non-Intel Quad (top of the range in its time) until it blows up and I can justify investing in something more up to date. It's working just fine atm. Of course I can't import AVCHD directly!
I'm using ClipWrap to convert my RX100's AVCHD (shooting at 50p 29m (PS) for max quality) to something that this older version of FCP can handle natively. I'm very impressed with ClipWrap (thanks focker) and have converted AVCHD (in 50p 50i 29m 17m etc) test clips into AIC, ProRes 422 and ProRes 422 HQ. I also have the option of DVCProHD and XDCAM EX and HD (as well as DV and HDV) which I have not tried.
The ProRes 422 HQ looks great and handles well. But the files are huge compared to AIC (and HQ is bigger than plain ProRes 422 o/c). The AIC also looks fine, but I haven't got round to doing any serious editing yet.
The guys at LAFCPUG.org (my favourite FCP geek site) are all saying AIC is old, go ProRes422 HQ as you definitely won't lose quality even though you won't be able to take advantage of all the other abilities of 422 HQ..
Thing is, will I lose anything using the older, simpler, smaller, tried and tested AIC for what I intend to do? - which is basically show the edited footage from hard-disk to its best ability on a 1920x1080p TV screen, burn DVDs (I'm not BluRay capable at the moment, but will in the future no doubt), and code some 640x360 stuff for the web.
So, go max with ProRes422HQ and cope with the enormous file sizes and inevitable slowness with an older Quad? Go AIC and maybe lose some quality? Split the difference and go ProRes422? Or try one of the other formats like DVCProHD?