Depends if you're getting the 21.5" or the 27".
But most important upgrade would probably be GFX for future-proofing it.
After that it'll be the CPU(but for most people, the default CPU will be more than enough so this upgrade really isn't recommended unless you're doing some processor-intensive tasks).
Harddrive/Fusion/SSD can always be added externally later.
RAM can easily be added later as well with the 27". If you have the 21.5" it can be added later but it is much more difficult.