No, it won't. The iPhone doesn't store contacts on the SIM. Keep all data on the computer and then the iPhone and iTunes will handle the transfer of the number.
It depends on how the phone was activated. If it was activated with new sim card, then your existing sim card will no longer work, and the one that comes with the new phone will.
This is going to be a little ridiculous to ask, but on my computer there are already a bunch of contacts in my address book because I changed all the screen names on my buddy list on iChat to peoples actual names which creates each of them as a contact in address book. I don't want these contacts to sync over to my phone. How do I sync only the contacts from my phone to my comp and no vice-versa.
Also when I get my new phone how do I sync only the contacts I want over, can I create a list and just sync that list?