Personaly I'd say go Linux. You're performance issues are going to deal with the speed of the communication between the servers. If they each have a dedicated NIC talking to the other then you won't notice any difference.
It's fairly easy just make sure you're MySQL box is up and running and has a port open for recieving MySQL calls (I don't know what the port number is check the docs). Then in your PHP scripts just add the host to be your database server. (i.e. db.mydomain.com or 192.168.0.? or 10.10.?.?, whatever your internal network setup is).