My advice - be careful.

Your criteria might limit your choices.

Best of my knowledge, Verizon does not offer GSM (at least not last time I checked).

You mioght consider either upgrading the phones - a quick call to TMobile might allow you to do this for free. Granted they won't be top-end devices.

You might consider Cingular but be careful - if you are a heavy data user their plans can get real pricey.

Sprint also has good family plans but then again no GSM...