You will have 400 GB of nominal storage space if you have 2x200GB drives but the way in which the drive manufacturers measure 1GB is 1000x1000x1000 bytes which is less than the actual 1024x1024x1024 which a GB is officially and in addition the formating will take some of the space which will reduce the apparent size, so you will see them as less than 200GB each. But both of these are the same for all drives so I would not worry about them.
The 8MB is a cache on the drive so although you will have 16MB in total it will be split between them and one will not be able to use the 8MB on the other.
As regards whether the other drives would be better, I will leave that to someone more experienced to say.
Raid 0 is so far as I am aware 'mirroring' which means that although you will have the 400GB, because everything is written to both drives, with the exception of the speed and security, you will effectively be operating with 200GB of storage space.
Hope this helps.