Wirelessly posted (8310: BlackBerry8310/4.2.2 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/102)
First off, goto your MESSAGES, press MENU, and hit OPTIONS. Go to general options, scroll to thr bottom and change 'SMS and EMAIL inboxes' to seperate. This will make it so you can view your SMS/MMS in a seperate icon on your desktop.
If yahoo mail is the only mail you are using, you can hide the default messages icon, and just use the Yahoo messages. You can do this by highlighting MESSAGES on your desktop, press menu, and slelect hide.
As for your SIM card question, you can use your old sim card without having any problems. But since they did give you a new sim card. You can transfer your addresses by putting in your old sim card into the phone, run Desktop Manager, and do a backup. Then insert your new sim card, and do a restore.
Good luck and PIN me I'd you have aby questions. PIN: 242DD5B5