He JP, I have experienced similar problems, and as it is, buying more RAM is pretty much the only option. You might be able to get away with setting the hash_max really low (10 or single digits), but then raytracing will take extremely long (or the computer will just hang without crashing).
Andreas -- Andreas Förster Dept of Biochem, Univ of Utah, 20N 1900E, #2460 Eccles Bldg. Salt Lake City, UT 84132, phone: 001.801.585.3919 home: 465 3rd Av, SLC, UT 84103, 001.801.364.0529 http://www.biochem.utah.edu/~andreas