Well the only reason that I can see for a person that knows about computers, and i really mean KNOWS, is that it can be spitting jets of fire 10 feet high from every single orifice that it possesses, and as long as it was not caused by a coke spill on your keyboard the Mac store will fix it for you for free.
Best suggestion I can give you is this, since you won't be building your PC (are you sure? with 2000 you can build a near super computer) and you want to avoid/minimize the chance of the same thing happening I suggest you figure out which Alienware computers are easiest to upgrade by yourself. Since if something does break then you will be able to replace it more easily later on.
On a side note, Blizzard never pushes PC tech with their games, they don't need flashy HD to cover lack of artistic skill, they have more of it than almost any company I know out there so no matter what PC you buy right now you will be good for D3.
@twroo
Besides start up processes look under Services and disable the ones that are extra. It will probably be full of Microsoft crap, but I have seen bullshit in there as well.