I can easily use more than 4GB worth of memory in one app. :P Then again I work with 3D animation
I stalled my poor workstation (dual Opteron) when i tried to over load it with a huge scene featuring over 20 million polygons :P with lots of ray tracing
Nice thing is XP 64bit and Lightwave64bit handled the extra load quite nicely, and nothing crashed (what you'd expect from a system that can handle several TB of memory (physical + virtual) :P). But my measily 2GB of memory definately starved the system (read: lots of paging to disk).