Thank you for excellent articles, great books and instant inspiration.
I believe that problems with slow performance of iPhoto are rather not related directly to the size of library but to amount of computations required for screen rendering. So you can have really big library and work efficiently just by reducing the amount of photos that iPhoto is rendering on the screen. The best approach is to sort photos by film rolls and then just then click the triangle next to a film roll to hide any photos you're not using.
Using this simple technique I was able to work efficiently with libraries of about 3GB on my PBG4 1GHz.
with best regards