[PERFORM] index performance on large tables with update and insert

I have a large table with about 2 million rows and it will keep growing...

I need to do update/inserts, and select as well.

An index will speed up the select, but it will slow down the updates.

Are all Postgres indexes ordered? i.e., with every update, the index pages will have to be physically reordered?

Does Postgres have any kind of non-ordered indexes (like Syabse's non-clustered index)?

What is the common way to take care of the performance issue when you have to do both update and select on the same large table?

Thanks,
Jessica