Well, seeing the case…. I’d dump the rear 80mm 120 adapter contraption, and just put a 120mm fan for exhaust. You said you could put a 120 intake on, do that. Looks like you could put a small side panel fan in, that’d be a good intake. Your PSU likely won’t be exhausting a lot, unless its really pushed, in which case your likely undersized. That would be balanced or slightly positive. Not much else you can do, PCI slot fans are marginally useful. So unless your prepared to start drilling out holes for new fans!?!?!?!. (top exhaust, bigger side intake)
I wouldn’t get too caught up in the pressure; the arguments revolve around keeping dust out of the case. With positive you need filters on all your intake fans and air flows out of the cases open holes and cracks keeping the dust at bay. With negative you need to seal up every crack and hole in the case so that air and dust don’t get sucked in that way, and you still need filters on your intakes.
I've decided to get another 120mm fan and make it do exaust but I'll be transfering the 80mm one to do a bit of work on the side pannel... :P I'm cheap like that. The thing is, I won't be doing any serious overclocking. I plan on pushing my e2160 to around 3.0/3.2 Ghz. Also, I'll be running a single HIS 4850 in this build (that I don't plan on overclocking just yet). That's about it, nothing too fancy :)