
Recently, i got a project which viewed in Full HD 1920×1080 resolution and also on standard 720×576. Apparently, AE CS3 which I’m using isn’t coming with double preview, both HD and SD. Luckily, AE CS5 has this feature. So, I made masks in CS5 and imported it into CS3. I’d love to work fully in CS5, but since i got plugins which worked in 32bit only, I found the guide is useful. If you are like me, who has project to be viewed on HD and SD resolution, perhaps this will also be useful for you. 
you can download the guides here.
Good luck with your projects