My Mac Pro received another 1GB RAM upgrade tonight. That puts it at 3GB total. Apple's Aperture just sucks up the RAM. Especially when dealing with the NEF (RAW) files from the Nikon D80.
The extra RAM has made a considerable difference. Aperture is snappier and I can listen to music while I play with my photos.
Doing a self-install of the RAM was a snap. Here's the instructions:
1) Power down the Mac Pro
2) Open the side panel
3) Pull out the top RAM riser
4) install two 512K RAM chips (from Apple)
5) Put the memory riser back, close the side panel and power-up