Jim, I have never worked with/on those computers, but looking at their manuals
http://downloads.dell.com/Manuals/all-products/esuprt_desktop/esuprt_dimension_desktops/dimension-9200_service%20manual_en-us.pdf
and
http://downloads.dell.com/Manuals/all-products/esuprt_desktop/esuprt_dimension_desktops/dimension-e520_owner's%20manual_en-us.pdf
it looks like the RAM M I G H T be interchangable: BUT without knowing the exact specifications of the memory modules it is impossible to know. Adding the E520 modules to the 9200 could very well cause the PC to Not function.
The forum can give you a better answer if you can post your PC specs. The RAM sticks/modules can be carefully removed from their slots and should have stickers on them that tell you what speed they run at (600, 800 etc) and what the various timings are. You should be able to find what processor is in your PC's by going to Dell.com and entering the service tag number found on (usually) the back of the PC.
The hard drive can be moved, but more then likely the HDD from the E520, with W8 on it, will NOT boot up in the 9200: This is because, someone correct me if I am wrong, the Operating system (W8) is "tied" to the motherboard of the 520 (assuming that the OS was installed by Dell as an Original Equipment install and not as a retail install. It Could be used as a storage drive for music, pictures, documents etc.
It is possible that you might gain a little speed from adding more RAM, but both systems are quite old and it might just be a better idea to save some money and replace them. If you have never worked with PC's trying to speed one up a bit by part swapping can turn into a real nighmare: Hell, even if you work on them often it can be a nightmare.
Good luck, I hope that this helps.