As I see it, the main advantage of a plug is its size and power consumption. You can certainly build your own system, maybe even for cheaper, but the size as well as wattage are what matters to most people.
I agree with you, in fact i am an sheevaplug user ( or I was and now I'msheevabrick user, but however )
I am not saying in any way that this system is better or worst than a guruplug server...
no intentions to make it a challenge between plug's and mini-itx
what I meant is that if someone can give me some tips about choosing a sheevaplug or a guruplug or the configuration posted above
considering tasks I mentioned, and I think that are the popular tasks we need on an home-server...
I may say that for my specific case, being a student with very little incomes, the most important matter for me is the cost of the system, then comes the power efficency and quietness.
I have no problem about space and CPU power must be enough to run those service i mentioned.