Adobe After Effects
Adobe After Effects CS4 (and CS3) spawns subprocesses -- one for each core -- when 'multiprocessing' is enabled in Preferences. Each of those subprocesses can grab up to 3GB of RAM. We typically use the Total Benchmark created by Brian Maffitt to benchmark with After Effects. If you run Activity Monitor during the rendering, you can not only observe the multiple subprocesses but you can observe how much real memory each of them is appropriating as the second phase of the project render progresses toward completion. When we had 16G installed, we observed 13GB of real memory in use.
As a general rule, the more cores you have and the memory you have, the better After Effects can "breathe."
Only certain functions/filters in Photoshop use multiple cores. A few examples of "MP aware" functions include Rotate, Gaussian Blur, Lighting Effects, Lens Flare, Pointilize, and Sharpen Edges.
As for memory usage, though you can only specify up to 3GB memory cache in the Performance Preference panel, Mac OS X is clever enough to grab unused memory as a virtual scratch volume instead before handing off the task to the actual scratch disk. If you are editing RAW photos with lots of layers and lots of history states, having the 8 memory slots in the 8-core Nehalem at dual-channel speeds can be better than 6 sticks running at triple-channel speeds. That's because slower memory transfers are better than really slow hard disk hits.