Sources inside Nokia Brasil have confirmed that test builds of the next Symbian iteration, namely Carla, are running on QA Nokia N8’s. It is yet unknown if the final Carla build will be released or not for the Nokia N8 and it’s likes.
The final build is not yet complete, so the final system requirements are still not very clear. It is possible that Carla will be made available for the 680MHz Symbian phones, but with a smaller feature set, since the current feature set seems to be running on the N8. It is my opinion that Carla will be for Belle as Anna was for S^3, so the impact on performance should be minimal. Donna, however would be a completely different story(if it ever launches).
On a second note, the same source said it is possible that Carla builds will be released to application developers pretty soon, to ensure application compatibility. This can only mean that the Nokia N8 successor that will be launched this month at MWC may come with the new Carla on board. Why else would Nokia release developer builds right? On the same note, it is possible that Carla will add support for 1080p video recording, since it seems that the N8 successor will be able to handle that resolution.