I work a desk job. If I bought the best video card that fit in the thing (it's a decent system, I design tools with Autocad Inventor 3D modeling software) then mined all night while I wasn't there, no power costs. THEN, surely it is very profitable...

Yeeaaa... you probably thought about getting some Nvidia Quadro or Tesla, eh?
Over 9000 GHash/s with one of those cards, fine quality - really nice.
Workstation cards like Nvidia Tesla M2090 are horrible for integer calculation like bitcoin mining. They will yield about 200-300 mhash/s but cost $7k to $20k dollars.
They have phenomenal performance for single and double precision floating point operations though (1 Nvidia Tesla M2090 card @ 1300gigaFLOPS equivalent to over 200 radeon 6990's)