You shouldn't have a problem wiring up the PS2 and the N64 to the PSU then! EDIT: So you're not gonna put a gamecube in it then, someone here is selling a gamecube and games for £45!
http://www.newegg.com/Product/Product.asp?Item=N82E16811144113 something with this kinda width but obviously a lot more height, would be substancially ideal, been googling around and found nothing yet, going to some computer fares and car boots soon, so am gonna be looking for an old box to play with maybe i'll get lucky got the n64 dismantled, and yeh my friend says it runs on 12v apparently, but i'll get my leccy friend to check it out dont wanna be going big-ba-da-boom just yet like mr willy wonka said, i thinks its gonna be coming off 2x psu's aswell as an n64 ps2 mini dvd-rw and cd-rw, 7 in 1, 2 sata's and the usual bumph. its gonna be a squeeze even in a big case <edit> and not forgetting a lot of cooling, i cant access the for sale forums </edit>
for the fan hope/open to suggestions bit: PSONE LCD with switch to alow switching between the pc's output and the ps2's?
Mini ps2 has a top loading DVD drive, and if I understand your design you've got the front of the ps2 with the front of the case, that means you cant get to the DVD drive
I am actually working on a mod like this right now. Mine is going to have one (maybe two) PCs, a modded Xbox, and a modded Gamecube. I am scabbing the case together from two or three older cases that I had laying around. Should be water cooled, and output to a single monitor and one set of speakers. The plan is to put it up in the worklogs as soon as I get something going.