If I take the SIM Card in my Cingular phone and put it in by BB in place of the one in there now, will both work, giving me my cell phone number on my BB.

Yes, it should work and worst case you can swap them back. Not that putting the sim card in your BB will buy you much but you can copy the phone numbers from your sim to the BB address book, so I guess that might be nice. A new 8700c comes with a 64K SIM, not sure what your phone is, probably won't matter as the BB really doesn't put anything on the SIM other than the phone number.