A WMR200 can accept up to 10 frequency channels (tap on the "CH" button) so effectively 10 sensors (excluding wind, rain, indoor sensors of course).
Ensure that the sensor is labelled as using the 3.0 protocol which is compatible with the WMR200. The Oregon Scientific website usually also lists which units a sensor is compatible with. Both of these sensors will work with your WMR200, but it's probably a good idea to get the 810 as it supports 10 different frequencies which does make it more flexible as you add more sensors to your setup ... and it seems to be cheaper than the older 800.
Given the price difference between the 800 and the 810, and I only need three more temp sensors it makes no sense to spend the extra for the 810. However, if in the future I wanted additional ones, do the frequencies conflict any so that i would then have to replace the 800's?
In other words, can I mix the 800 and 810, with three 800's and seven 810's? There is enough flexibility in the channel selection, right?
Thanks