Blowing down is normally better, but it depends on case airflow patterns. I tried both on my last system and didn't really notice a difference in CPU temps. Suck made my case temps a couple degrees lower though, probably because the case exhaust fan did a better job of picking up the warm air coming off of the CPU coolers.
In some cases sucking air up might be preferable even if it results in slightly higher CPU temps. Suppose you've got a side vent & shroud, setting it to suck would blow the hot air out of the case. That'd probably hurt your OC on the proc vs. sucking in cool outside air through the shroud, but might help your vid card.
I'd say just try both and see which one works better for you.
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