What type of cooler are your 5830's using? If it's a turbine-type cooler with intake on the rear (the end near your drive cages) of the card, I'd position it blowing into the cards there. You'll be able to turn your GPU fans way down, it may actually extend their lifespan.
If it's a 'blow everywhere' type like the Saffire 5830's that exhaust out both ends of the card, then positioning so it aids the intake fans will have a similar effect. I hate this cooler design for mining, as it's dumping heat all over the inside of your case.
+1 on researching pos vs neg pressure - normally it wouldn't make a big difference but you're going to be pushing a huge amount of air. It may be worth trying to setup a postiive pressure system, theoretically allowing the GPUs to more easily pull air through the fans, across the heatsink, and mostly exhausting heat out the back of your case where it belongs.
Experimentation is really the key, play with positioning until your GPUs get chilly, and fly with that.