T-H-E,
the 
...
The difference in air temp from the bottom of your case to the top (from the outside) is going to be very small. It's always worth pulling 'out of case' air in whether it's from the top or the bottom...
It's suposed to be better to have more fans blowing in 'cause it stops dust being drawn in through every crack or hole. That's the only reason. Whatever produces the most airflow through the case is best, regardless of whether it creates positive or negative case pressure...
I'd concentrate on fitting everything into your case as neatly as possible first, otherwise you'll be chasing that 'degree of performance gain' forever!, and 2DegC does'nt
usualy make a damn to CPU performance/overclocks
...
PS: I'd use a single heatercore. There's no reason a single heatercore should'nt 'dispose' of a CPU & GPU's & HD's heat. If you're getting awful temps
with a HC then I'd look elswhere for the problem...