pgsql and large tables

I've got a large database, currently about 40 million rows in the biggest table. I'm trying to set it up with PostgreSQL 7.1.3, but I've got some questions...

I initially created the table and with a primary key (5 fields: char(4), varchar(32), date, int, int) and a couple additional indexes, one that just changes the order of the primary key, and one that sorts on the date. Then I imported 40 million rows and tried some queries, but discovered that none of my queries were using indexes, causing them to take forever.

So I read somewhere in the archives thatyou need to VACUUM a table regularly for indexes to work properly. I tried that, but aborted after about 5 hours. I can't use pgsql if I have to take the db down for more than 10-15 minutes a day.

Then I read somewhere else that you should drop your indexes before VACUUMing and re-create them afterwards. I tried that, and VACUUM finished in about 10 minutes. Kewl... but now I've been trying to recreate my primary key for the last 18 hours...not so good.

Should I have dropped all indexes *except* for the primary? or would VACUUM still take forever that way? Should I make an artificial primary key with a serial type to simplify things? Anyone have any hints at all for me?

thanks,
Partap Davis