Ultra-mobile sff gaming/all purpose rig ~$600?

Hello all,

I'm not at all new to the topic of building computers, but I don't know much about small form factor hardware, so here is my question/concept:

I have an office, and a home, I like to bring a computer back and forth between the two because A) I don't know how safe the computer would be alone in the shared office-space, and B) I want something powerful since I game a good bit (and I would like a second gaming rig around the house) and C) my job (physics grad student) can involve some CPU-intensive simulations, etc at times.

Goals:
-fit in a backpack/laptop case
-Be significantly more powerful than your average laptop.
-no monitor necessary, I'll leave a cheap one at the office.

Questions:
mini-itx vs matx - obviously mini itx is smaller, but more expensive in general. I also have a phenom 965 BE and matx board to go with it - so I could re-purpose those parts, underclock the chip, and buy a new mobo/proc for my main rig. On the other hand, I could just buy new chip/mobo, no difference really, would need one in either case anyway - but I assume I could get more for the same price if I'm buying a new atx board instead of mitx - haven't looked prices on these up much yet.

case recommendations:
something that will fit a big shuriken perhaps ~3-4in thick? but with minimal dimensions otherwise (slim). This is my main concern, finding a nice slim case is a pain.

Recommendations on mobo/case/psu all appreciated. If I buy a new proc/mobo I will probably go with an intel (2500k I guess, but maybe something lower-power if necessary) - Obviously the AMD APUs are an option if I can't make a decent gfx card fit in one of these cases, but with a pcie ribbon, and perhaps some other adapters and rigging, I might try to throw a gtx 550 or something in there, depending on what the psu can handle.

What are your experiences?