ichat newbie question

Hello everyone, new Mac owner here. Made the leap a month ago and I already know I will never go back. I love this machine (iMac 2.0 w/ 1 gb and 20"). Talked my Dad into making the switch also, we have been video chatting and I have one question I can't seem to figure out:
How do you switch back to video chat after screen sharing w/ out starting a new session? (the guy in the leopard promo vid can do it, is it bs?) Also, what level of .mac membership is required after the free trial to keep ichat running w/ screen sharing?
While I'm at it:
what is the min. external hd needed for time machine? I have a 100 gb, is this enough?
One more:
When burning a dvd the encoding takes forever (sometimes 2 hrs. for a full length movie) How much would going to 3 or 4 gb of ram help this? Everything else runs great on 1 gb.
Wheres a good place to buy discount ram that will still be reliable? How much should I spend?
Thanks in advance for your help.

There are several places for memory. You can start at Newegg.

I don't believe screen sharing requires a .mac subscription, just leopard, I don't quite know how to stop screen sharing without starting a new session, but i think it may be something in your buddy list. Umm... the external HDD should be bigger than your internal HDD for best results (that's just my opinion)
Not sure if the mem will boost the encoding speed, i always thought that was more to do with the processor and burner. but i could be wrong.

best place to buy ram would probably be www.newegg.com

you can get 4 gigs for less than $100 there. which is an amazing deal.