Hmm not sure about that RAM specifically, but would be interested to know what case (1U, 2U, 4U and make/model etc..) you are using. As far as cooling the stock Xeon coolers are usually just fine assuming installed/seated properly etc.
The biggest thing to watch for with smaller form factor cases such as 2U is to make sure the case has all the fans that are required and that cables are routed in their proper spots (IE if you have a FAN Wall, make sure to run HDD cables, front panel cables etc on the right side and towards the bottom side, etc). If it comes with plastic "channels" for the cpu's make sure those are installed and just in general make sure the air is flowing throughout the case as it doesn't take much to obstruct the airflow and cause overheating. You would also want to take note of the environment the server is going into and how the ambient temperature is, the cooler the air being drawn in, the easier the heat inside will dissipate (larger delta T).
Alright.....I think I have determined that it is an OS problem + Fan problem
The OS, which is Ubuntu 7.10 is giving out this problem, which freezes around 15 min after power on.
The reason I think could be my profiles, since I keep my /home, and formate and install /.
It can also be the nvidia driver. When I was using the live CD during the install, it never give this problem, and after installation of the OS and the nvidia driver, this problem started happen.
CPU Fan is also a cause, since it is not blowing enough air to the memory steak to cool them down.
In fact, the fan can not cover all the memory in the slots, I think it is just because the fans are for ranks, not for tower cases (which is Cooler Master Cosmos 1000 I am using)
So far, I installed ubuntu 8.10 RC, seems working fine without NVdia driver loaded. I also took 2 mem steak out from the box to make sure that they can be covered by the fan