"-B 1000" should work fine, as 16KB * 1000 = 16MB, which is 1/2 the shmem default that has shipped with RH at least for a long, long time. It's only if you go above that level that you should run into problems.
Then, again, my two latest "vanilla" installs of RH were 6.1 and 6.2 respectively, so perhaps with older versions there was a problem. Not sure.
In 6.1 (maybe 6.0) you can change the shmem size without a kernel recompile but you can't change the default allocation addresses, which Postgres uses for portability reasons (I keep meaning to suggest a command-line switch to override these values).
