If you will be using the BB on BES, get the BES/BIS Unlimited-Which is $44.99/month, and get the messaging started (200 txt msg) for $4.99. If you're only using BIS, get the BIS Unlimited for $29.99/month and the same txt msg plan as above. Both of the BB data plans give you unlimited web,PIN's,emails and IM's (on googletalk,etc.).
I have the 8700c with unlimited BES and the 200 txt msg plan, and I love it.