To Suck, or to Blow ????

I was wondering if it would be better to have the fan on the HS sucking or blowing.  I have an exhaust fan right above the cpu mounted on the case and i've read somewhere that having the cpu fan pulling air away from the heatsink then allows the the hot air to be pulled out by the exhaust fan resulting in more effective cooling of the cpu.  R

ight now i have my cpu fan blowing air onto the heatsink and have idle temps of about 45 degrees celsius and around 50-51 under load, do these temps sound normal for a AthlonXP 2100+ that has not been overclocked yet?