If you are using any ot them for serious gaming, then you may need to look at boxes (from the same set of manufacturers) that are optimized for gaming.

If any of them is being used as a file server with heavy I/O (reading and writing of files) you may want to consider gigabit ethernet. Since they are all media center types, you need to also consider if they are transferring streaming content among themselves.

I realize that this doesn’t answer your question as much as it asks you for more information – but if you want the best performance for the money, you have to be specific about what sort of “performances” you’re specifically going to be running.

Bob