You do realize the Conroe CPU only uses 65W or power, compared to the previous Prescotts 135W (or more)?
620W power supply is really over the top. A 500W will be more than ample. Just FYI.
Cases; I like the Antec line. Sonata II is nice and quiet, and reasonably priced. The 'P' cases from Antec are faily expensive but very nice boxes.
You do not need a lot of fans. The HIS card you are getting exhausts the heat out of the case. The Conroe runs very cool (no higher than 40C under load).
The RAM will be the hottest components you have. For that I recommend mounting a dead air fan in the case, just over the RAM. It will insure the RAM gets good airflow and will also move air over the backside of the video card.
A decent 120MM exhaust fan will take care of the rest.