For a rad you will need a fan that can generate a good amount of 'static pressure', meaning it pushes air forcefully enough to penetrate denser material, such as a heatsink or a radiator.
Its quite important because many intake and exhaust fans are designed to push lots of air but are very 'weak' when it comes to penetration, a no-no for rads.
Just gently caressing a rad with a strong breeze will not cool it as well as a fan that can push the air further in, even if it means less air. Moreover, I assume you are going to mount the fan pretty much on the rad, so once again static pressure is king, you need the extra push to make it work well. You might also consider a push-pull set up depending on your case and rad for some extra performance.
If all of the statistics are not available when considering a fan on the site, do a search for reviews they generally go more in depth and can help you choose the fan that generates lots of static pressure -and- airflow, but when putting the fan point-blank at something like a rad the former is more crucial.
- Ordokai
