With a non-overclocked system it isn't required, as MadAdmiral stated. However, you have left the door open that you do plan to overclock some time down the road, so I would get the HSF now. It will provide better cooling (AMD stock fans aren't the best) and will keep you from havig to remove your MOBO down the road to install an HSF. If you can budget it in the beginning, I highly recommend it to save you the hassle of installing one later on.
Dude i have been going through these builds for a while. you basicly just stated my whole build before i switched to a 5850. If you want to save some money you can get a cheaper burner on newegg, just look.