Each case is different (plus factor in the different fan's performance and characteristics).
I found what worked best for my case/components by trying different configurations, and then seeing the results. In my case, a "cross-flow" fan (bottom mount) helped my temperatures a bit, but it wasn't a big difference.
Edit:
I also meant to mirror what ClockHound stated on the importance of the components being cooled. Unless you have RAM heavily overclocked, it really isn't difficult to keep it well within it's rated specs. In fact, a lot of RAM with heatsinks are mostly cosmetic, and really aren't needed at all.