Funny enough, still photography is my main "creative" use for Macs. Most of my digital originals pass through my MacBook Pro, and most of those come from a Nikon D800. It spits out 36mp files-RAWs are 60-80mb and JPEGs are 15-20mb.
I handle MUCH larger files on my dual 2.7, though, and it actually handles them pretty gracefully. For those, I either start with a 35mm film strip or slide and scan it in a Nikon Coolscan V, or for any other format(as small as 110 and as large as 4x5, but mostly 120) I use an Epson V700. A typical medium format scan for me is about 14,000 pixels on each dimension, and since they are all "real" pixels and not Bayer interpolated a JPEG runs 40mb or so. You do the math on 4x5 at 6200ppi or even 4000ppi.
Pretty much all I do in terms of PP work on the G5 is dust spotting in CS4, although I occasionally do some distortion correction. The G5 lets me scroll around big files with no lag or other issues-it's as fast to use as my MBP.
Funny enough, a year or so back I got into a "debate" with someone on another forum who claimed that his 1ghz TiBook was "totally inadequate" for digital photography work in 2004. At the time, my main DSLR was a Nikon D70s, a camera that came out I think in 2004 or 2005. Just to prove a point, I pulled 100 shots from it into Lightroom 2 on my 1ghz Ti, and had zero issues working them up. The only thing that took a bit longer was importing them(I did it via a Cardbus CF adapter, as opposed to the USB 3.0 reader that I use with my MBP), but once in most of the bottleneck was the fact that some of the buttons are in different places in LR2 vs. LR6

. I'm not anxious to process D800 images on that computer(or big film scans) but at least with cameras contemporary to the camera it's fine.