Like others have said, expand the RAM in that system to the max, update to the most recent OS X that can run on it, and fit a larger drive (or SSD plus external hard drive). That will solve all your problems. Optionally move to Lightroom 4 if you so choose, although I know a number of folks who prefer Aperture 3 for M9 raw files. (I prefer Lightroom.)
M9 raw files are large and require lots of ram and disk space to work with efficiently.
For my raw image processing, I run a latest generation Mac mini with 16G RAM, 1T internal drive, and i7 Quad Core 2.6Ghz processor. I have all the image files on external hard drives, only the LR catalog folder is on the internal drive. It's super fast and does a terrific job. That's about a $1200 computer, plus keyboard/mouse/display/external drive costs, but well well worth it for any kind of serious photographic work. And it's simple to set up and use.
G