Sorry, I meant assuming you're changing to a HT2 crank.
There's no adapter involved in terms of them fitting your BB, external bearing BBs are for the same width shell as an isis/square taper/annoying Sugino taper BB, they thread straight on and the majority are BSA threaded. If you're running a HT2 crank then you just need a set with a 24mm ID. Shimano do several levels that all work fine, or you can go for the bling with Phil Wood, Hope, Wheels Manufacturing, Chris King etc. The benefit of those BBs is you're only changing the bearings when needed, the cups stay in the frame, so it's less waste and less wear and tear on the threads (although that second point really doesn't apply much if you're installing things properly). The Shimano Ultegra and DA work just as well as those fancy ones and I think they're a little lighter as the cup and bearings are smaller. This has an impact on longevity, especially if you're riding in poor conditions. It's the Same for Campag, but you need their cups (they make them in BSA/English threading).
If you're going to look at different cranks (Rotor, FSA etc.) then it's the axle diameter and ID you need to pay attention to, because many brands will use different standards.
In terms of adapters, these are usually only needed if you're going to run something like a Hope of Chris King BB with a GXP crank. The axle on a GXP crank is taper from 24 to 22mm, so rather than make loads of specific cups, they just made an adapter to shim the axle. Personally I'd stay away from doing this (I run a GXP crank on a fixed gear I have and I've found they can be noisy).
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