3500+ and 3800+ have just been launched this monday, so I don't think there's yet anyone who can tell you how they overclock. I believe the 3500+ should be the solution, but if you have money to spend, the 3800+ may as well do it. Are you building a new system, or do you have the rest of the components in hand?
I believe one can pretty safely assume that the limit for overclocking any A64 130nm CPU is ~2.6GHz. That's just how much AMD's design can give @ 130nm, pretty much like ~2.2GHz was the limit for SocketA CPUs (mobile Bartons don't count!).