The Vista OEM editions only allow you to associate your Vista copy with one device. You cannot transfer your license to another device. This means if you upgrade the motherboard, this is classified as another device and so you cannot install Vista on this machine. Although as has been stated in the thread, you can transfer the license by saying your replacing a broken motherboard, this is technically outside the scope of the EULA.
Now with respect to AMD and Intel in the licensing scenario, i believe the later Intel (native) quadcores will use Socket B. Therefore, if you buy a S775 platform, you will not be able to upgrade to these quadcores, unless you buy another motherboard, hence, the windows OEM license does not allow this. However, if you were to go AMD, their native quadcores (i believe) can still be utilised on socket AM2, hence you should be able to install Vista OEM on this machine and still be within the terms of the EULA.
The main difference with OEM and retail is that the retail license is transferable and can be used on unlimited number of devices, although only one device at a time. THe OEM license is not transferable but can be reinstalled an unlimited (i think) number of times on the SAME device that it was intiially activated on.