I am looking at the 1199 X51 only upgrade is a blue-ray drive for movies. My only problem I have is: is it upgradeable, I wouldn't think so with as thin as it is, also can it handle a higher wattage external power supply, probably around 500-600 watts so If I can upgrade the video card I can get a higher power one.
As at $1200 gaming PC it's a fail. A PC in a standard case with a reasonable power supply will crush it for less money. The video card is http://www.anandtech.com/show/6181/nvidia-quietly-relea... OK, but not leading edge. I wouldn't assume the case can handle something with higher heat generation even if you figured out how to get a higher wattage PSU in place.
As a small PC with good components its a win with people willing to make trade-off to get the small case size. Buying this PC (or a laptop or an all-in-one) with intent to upgrade is probably not a good strategy.
It's upgradable but your limited on what you can do, obviously with that form factor/size you won't be doing SLI/Crossfire and cramming in multiple HDDs.
However so far the best it can do for GPU upgrades currently is the GTX 670, Galaxy Edition would be the best one for it.