I spend a lot of time experimenting and working with pretty high MP images, as I have been researching approaches and algorithms for extreme digital upscaling of RAW images. I regularly take my original 12.2mp images to multi-foot dimensions comprised of anywhere from 220 megapixels (55x44" print @ 300PPI) to 2.4 gigapixels (96x72" print @ 600PPI.) I use photoshop for viewing my work.

In my experience, with a system that has 12 gigs of DDR3 triple channel memory, Photoshop is able to handle images several hundred megapixels in size pretty darn well. When I get up to around 600-700mp or so, things start to slow down (this is when all my physical memory is consumed, and swap usage starts.) Working with gigapixel images ultimately means that you just start churning on disk more. I recently upgraded to an SSD drive for my primary/swap drive, and the performance of working with 800mp or larger images has improved considerably, however it is best if you aren't running other apps when working with gigapixel images. It should also be noted that any processing you apply to a gigapixel image is going to take some time, regardless of what that processing is...and even with a very fast system.