I will preface my opinion with the fact that I know a little bit about how processors work, so my statement may be right or wrong.
That being said, the Lumia 800 only had a single core processor while the 8S has a dual core processor. While the clock speed may be lower on the 8S, it will still be plenty quick because it can divide intensive tasks between the two cores.
You'll enjoy a better app selection in future and much better performance right now (really fast loading times for apps and no more laggy swiping when you first launch the app). Keep in mind the low amount of internal storage on the 8S though, and it seems you will only be able to install apps to that storage. I think you should wait for a review of it so you can work out how much storage is available after formatting and the OS install, if it's like 1GB it might be a pretty crap experience.
It has Micro-SD support so you will be able to expand the storage, the screen is bigger and so it's the battery, a way better processor and Windows Phone 8. There's an odd chance that the camera might be better also.
A worthy upgrade in my opinion.
Last edited by Winterfang; 11-04-2012 at 10:17 AM.