Performance of 32-bit cpu machines will rapidly decline when the database size exceed 5 gig. You can run 30 gig database on 32-bit cpu but it will not be very effective. There is limitation imposed on the amount of RAM (maximum of 2 gig only), limitation on the file system sizes and also on the operating system. The operating system on 32-bit cpu will be "bumping it's head" too many times on the "32-bit ceiling".
If you need to use extremely large databases (greater than 5 gigabytes), it is strongly advised that you use 64-bit machines like Digital Alpha cpu, Sun Ultra-sparc 64-bit cpu, Silicon graphics 64-bit cpu, upcoming Intel Merced IA-64 cpu, HPUX 64bit machines, IBM 64-bit machines. Compile PostgreSQL under 64-bit cpu and it can support huge databases and large queries. Performance of PostgreSQL for queries on large tables and databases will be several times faster than PostgreSQL on 32-bit cpu machines. Advantage of 64-bit machines are that you get very large memory addressing space and the operating system can support very large file-systems, provide better performance with large databases, support much larger memory(RAM), have more capabilities etc..