actually, I think your friend has it backward.
I doubt that it makes more than a very marginal difference, but the same rule that says if you suck the hot air out cold air will be drawn in, also says that if you force air in it will find a way out.
the only real advantage of one way over the other is that if you force air in you will have a very-slightly higher pressure in the case, which would actually aid in how fast heat could transfer from the components to the air.
like I said though, I doubt the difference would even be measurable.
what would make a much more noticeable difference is to make sure you are pushing air in low in the case and venting high.
also if you can let air vent near high-heat components their heat won't add to your case temp.
if it's your DVD drive thats heating you up, and you have an empty slot I'd put the drive in the second slot from the top and use the top slot for a vent, that way the heat goes directly out of the case.
I hooked up a small window ac unit to my case. my CPU and MB temps stay below 85F most of the time, and tops out at 102F in gaming.
ps, bigger cases run cooler. the last one I bought was a full tower, very easy to keep cool and a lot of places to push air in and let it out.
