Not many of Sprint's phones are dual CDMA and GSM (meaning they also have a SIM card slot and can do GSM abroad). Those that do are smartphones that cost more and have more expensive data plans. These models are the Samsung IP830w, BlackBerry 8830 and Samsung Ace. They don't always have quad bands enabled to prevent them from being used in the US on GSM networks (they have only the overseas 900 and 1800MHz GSM bands).
When you renew with a new 2 year plan, they sign you up for whatever currently offered voice and data plan you like. That means if you have a really old plan that no longer exists, they can't sign you up with that one again. So make sure you're getting a comparable plan at a price you like.
If you're tethering a great deal, you will probably find EVDO a fantastic upgrade-- it's way faster than 1xRTT. I'd suggest getting a phone that has this feature.
For reception, Motorola is generally king. The RAZR VE20 has very good reception and EVDO. Like all recent cell phones, it has a camera and Bluetooth.