Marc, I am on just a regular price $39/month (or whatever) Cingular plan regardless of having the quad band phone, then when I have overseas calls those are billed by the minute at the price listed on the Cingular website for the relevant country. So you would not have a more expensive regular plan just because you had a quad band (or tri band) phone.

I'm not certain about the sim card question. I have heard of people changing cards to go overseas, but that was before I found out about the quad band phone. For me, a new phone (which I needed anyway) was the easiest route. Also, my phone was not particularly expensive -- about $150 if I remember correctly, with a $50 rebate available at that time.