Here is more information about how to run pgbench. http://www.westnet.com/~gsmith/conte...ch-scaling.htm
Basically, run with a small size and run for a very long time (minutes) to get accurate results for comparing CPU/memory system. Be aware of what is limiting the speed, the cpu/memory or the disk i/o?
It could be used for disk i/o testing with larger databases, and that would be interesting, especially if you can get some real raid cards (the expensive ones)