If I were planning to do what you are, I would call Alienware first, and make sure the motherboard will support that new CPU before doing that. It should work, but the Alienware machine is a custom build, and you never know if they have cut some corners to save pennies that might affect you on upgrades.
One other thing to ask about is heat. An I5 has 4 cores, and will produce more heat under a heavy load than a 2 core I3.