Shady06's suggestion of the Zalman 7000 or a Thermalright SLK900 or SLK947 are the way to go. These are currently the "top dogs" for air cooling the P4. The Zalman700 comes complete with a fan. With the Thermalright's you get to choose whatever 92mm or 80mm fan that you want to put on them. You can go from mild (a quiet Panaflo that will give you sufficient cooling and very low noise) to wild (one of those silly Vantec Tornados or Delta fans that will give you the best possible cooling with the most possible noise), or anywhere inbetween. Either the Zalman or the Thermalrights will do a great job, it's kind of a "flip a coin" type of situation.
I'm using an SLK900U + a 92mm Panaflo L1A on my 2.4C and it keeps it plenty cool without hardly any noise.