XPS Gen 3 Hard Drive Capacity Limits

I am looking for some tech info on how much storage space I can cram in the Gen 3.  I am going to keep my raptors and replace my wd 400 gb sata with a seagate 1.5 tb.  I was then going to enclose my 400 sata (external case) and use a external sata connection to main board (position 3).  Can this machine except 2tb of hard drives on its sata bus?  ... or am i limited in my storage capacity?

I am also looking at upgrading my video card (ATI radeon 800xt) to the fastest All-n-One which th power supply can muster... What is the max wattage of the power supply?  350? 400? 450? 

The ultimate goal is to create a home server out of this rig until funds are available for a new one.