I've been working on the poster that's newly linked in the sidebar. Initially this was at 72 dpi (since most of it is path based), which resulted in a 3024 x 2304 pixel image. My 1 GHz TiBook handled that fine, but when I switched to 288 dpi for the final tweaks (adding images and what not), it really began to crawl. You'd think with a gig of RAM a 12096 x 9216 pixel image would be OK to handle, but it seemed like I crossed over some invisible boundary and even basic things like rendering type required a progress bar. Saving took 5 minutes and let's not even speak of scaling and PDF conversion (I had to back to 72 dpi a couple of times so I could print out a quick version for typo checking). The "efficiency" meter was usually at 100%, so I don't know what exactly the problem was, but it was very frustrating.
If I did this kind of stuff for a living, it'd make a great excuse to get a (dual) G5. Unfortunately the best I can do at the moment is "decreased compile times", but that's still a bit of a stretch.