Determine now if you can whether they use CDMA or GSM cellular technology. CDMA is pretty much not used outside of one carrier (Verizon) in the US. Mostly not used in Europe by any carrier there. If yours has a SIM chip, it is GSM.
That means your iPad 2 would have to be of the Verizon type which works on CDMA also. In other words, don't buy the AT&T 3G model.