1) If your current CPU and mobo are skt939, then you will need your new mobo to be skt939 as well. I have a Gigabyte K8N-SLI
. I have found it to be reliable, easy to setup. Not good for overclocking, but you mentioned that that was not important. It is also inexpensive.
2) Skt939 Asus mobos are pretty notorious for their chipset fan dying. Generally it didn't cause overheating, but I suppose it is possible. Read my guide on random reboots as there is a lot of similarities in troubleshooting for this situation.
3) As a general rule, yes. You will have to format and reinstall Windows.