Well, there's that rule of thumb which says you should have at least as much lift in your BCD as you have weights to get you down.
I've always found that in full kit other than a suit (ie t-shirt in the swimming pool) I need no weight. Add a drysuit and nothing else, I suddenly need 11kg to get down. So the drysuit adds 11kg of buoyancy.
So in the event of a total drysuit failure, I would loose 11kg of buoyancy, so I need an absolute minimum of 11kg from my BCD.
Since my twin 7's hold 4kg of air, I would realistically need 15kg to be on the safe side.
I use the Buddy Tekwing, which supplies more than 20kg, so I figure I'm safe enough
So I would say, add the weight of air in the biggest twinset you're thinking of buying to the amount of weight you need to carry when drysuited, and make sure your BCD has that much.
Incidentally, if you want another way to work out cylinder buoyancies and gas weights, you can always use the one I wrote, which also takes what mix of gas you're using into account..
http://www.oneandoneis2.demon.co.uk/DiveWeb/cylbuoy.xls
(Edited by Dominic at 8:28 am on July 24, 2002)