I purchased a retail copy of Snow Leopard for my son's mac as soon as it was released. Installed and all fine. I have since bought a new iMac for myself and it came with Leopard but I can obtain a version of Snow Leopard for a nominal sum. 1/ Can I use the retail DVD to upgrade my iMac? 2/ If I go for the upgrade version for my iMac is it then only useable on my imac?