The extra megapixels are nice to have if you crop too. Beware though that some TIFF files can become huge monsters. One I have out of it in post in CS6 has grown to 400+ MB in size!
But, you can also crop into an image so much that you almost feel like you've shot with 50 more mm of lens. My avitar photo is a massively cropped image from a 105mm 2.8 portrait taken from about 10' away. I only zoomed in that tight initially to see how sharp the eyes were but to my surprise there was actually an amazing amount of detail from the sun-light coming through the courtyard and arched columns.
Also, the RAW files are different in the 800 than the 700 and you can't process the 800's RAW files in Lightroom 3.6. I needed to upgrade to 4.0 but alas, my computer will need to be upgraded to allow for that. Humm. Now I'm having to convert them first then import to LR, an extra and time consuming step. Long story short, make sure you have the technical platform in place for the 800 files, or at least budget for immediate upgrades.