i also have a 20gb hard drive, and the partition setup i used is as follows:
hdb1 / 6119.61mb
hdb2 swap 526.42mb
hdb3 /home 7649.52mb
hdb4 /usr 6119.61mb.
I'm sure there are many other ways you can partition your hard drive, the only way you can be satisfied with the way you partition a drive is by playing around with it. I'm sure this setup is not very well planned out but i'm not yet a guru