The largest map that I've worked on has been 10,000 by 7500 on a Mac with 4GB or Ram and using a 32 bit version pf PS CS4. It was slow, and a little jittery, but worked fine. One thing I found to be a huge help was having an external swap drive - in this case a fast (7200rpm) drive attached by firewire. That makes a large difference for large file sizes. Also, turning off maximise compatibility when you save it helps speed up the save time to something reasonable.