CPU fans always blow air into a heatsink. This way you have the coldest air passing through to dissipate the most amount of heat. Which in return the air exits the sides of the heatsink and cools you mobo and RAM down.
Actually, my old Dell pulled air from the heatsink. It had a metal heatsink on the CPU and a case fan mounted on the back pulling air out. Only fan in that thing, no telling what the temperatures were like... It still works though! Though now I have a fan on the heatsink, haha.
Yeah, Dell uses passive cooling on most of there systems. It just pulls cold air from the side over the heatsink and blows the hot air out the back. I have seen some passive cpu coolers that have a fan on each side. One pushing cold air in and one pulling the hot air out.
some people have gotten better results on a XP-120 with the fan blowing up instead of down. Its all about experimentation. And im using the same heatsink, Zalman 7000B, but a different fan. I took the fan out and modded it.