The image is currently 32.444 x 48.667 at 72 pixels/inch. What is the best way to size it to 12 x 18 in PS CS2?

Some places (like smugmug/EZPrints) have different resolutions depending on final print size. 300 ppi is "standard" photo quality, but anything larger than 10x15, they do at 250, since you'll view it from farther away.

What I would do is use the crop tool, set it to 12 in, 18 in, and 300 (ppi), and crop it. You can also use the Image | Image Size dialog and play with those settings.