Aperture performance/Library size
More RAM may help. Read all about previews here:
Keep projects reasonably small. Rebuild the database index (quick and easy):
Or rebuild the entire database by starting with command and option held down (slow but thorough).
I suspect that Leopard and the next rev of Aperture will speed things up, but we don't have those yet.
Aperture articles > http://www.bagelturf.com/