Should air be forced down onto, or drawn up and away from the CPU?

never really thought about it before..

Well it depends on the person's cooling setup. The way they have their case fans setup, someone with a fan pulling air away from the heatsink could work better than pushing air onto it, some heatsinks are even designed for this. Most people here get better temps with it blowing onto the heatsink, but you could try the other way. See if putting it one way will give you better temps than the other way. If not, just put it back the way it was.

thiking about the physics of it, drawing air up should work better, as if you blow air down onto it, the air will heat up surrounding components so some heat will not b dissipated far away from the cpu, but then again, unless u have only got air coming in at the botton of the heatsink then the air just directly under and around it will b drawn through, so if your heatsink has fins taht are open at the side all the way down then blowing air downwards [theoretically] would b better, but i think if you have it setup right then drawing air up should b better.

also if u draw the air upwards then you can just have a duct going to the side of the case to get rid of stale air as opposed to haveing to rely on an exhaust fan. this setup would also b aided by a HIGH positive pressure as the air could b forced through as well as being sucked through.

but i'm just speaking from theory, so in the real world it is prolly different!

Yeah basically is depends on the heatsink some are designed to have the hot air drawn away others are designed to have cool air blow over them all depends on the heatsink really but try it both ways and see wich way works best for you.