for what it's worth. i had a mac mini with 1gb of ram, and i was able to have itunes, safari, Mail, and microsoft word open all at the same time, and I never had any slowdown compared to when the programs ran just by themselves.
1gb is enough.
but read the other threads where they talk about ddr ram and what advantages there are to getting 2 sticks of 512 (and saving $80) vs getting 1 stick of 1gb with room for later.
the answers to all your questions are in these threads. I know this because I started from practically no knowledge of the mac 6 months ago, and after I started reading the threads (old and new) i felt like i had been using macs for a long.