I've done a quick seach but did not find any clear info on how to use
available memory more effectively. I am using FB 2 beta on a
x86/suse-linux with 2GB of mem and in my tests increasing
DefaultDbCachePages seemed had no effect (65535, pagesize 4k) on the
time of some large queries (in fact, the actual database is
small ~40 MB so I imagined some change).

Other DBs recommend tweaking shmmax and shmall, but I also didn't see
any reference to this anywhere.