Fragmenting tables in postgres

Hi,
Anyone tried fragmenting tables into multiple sub tables 
transparently through Postgres rewrite rules ? I'm having 
a table with 200,000 rows with varchar columns and noticed 
that updates,inserts take a lot longer time compared to a 
few rows in the same table. I have a lot of memory in my 
machine like 2Gig and 600,000 buffers. 

I really appreciate any pointers.

Karthik Guruswamy