ok, i'll elaborate on how you can use iPhoto Library Manager (iPLM)...
get the app:
versiontracker link
install.
now, the easiest way to split the existing library without messing up all the comments, albums, etc. is to simply duplicate the iPhoto Library folder under User -> Pictures.
after duplicating, use iPLM to select the duplicate folder as default, launch iPhoto, trim the library/albums until the data in the library is comfortably below your RAM.
for example, i use a 3MP camera at the highest settings, which results in ~1 MB per photo. i have 640 MB ram, so i usually keep around 200 to 300 pictures per split library, which, with comments and enhancements made to photos, results in about 500 MB library. i split chronologically, like 2003, 2002, etc.
repeat until all of your pictures are split.
it's a wonderful program, give it a try. it will give run your iPhoto much, much smoother than if you kept your library in one piece.