The OEM license is only valid on the original PC as the OEM can only support hardware which is theirs. When you change the motherboard, you essentially have a non-Dell motherboard and the system locked preinstallation mechanism will reject the system as theres a non-Dell BIOS. You can still change the CPU (providing it fits in the motherboard) and RAM. Probably one thing which will help you a lot is the installation of Windows 7 64 Bit on a Solid State Drive.
I've got a Dell Vostro 220 which is about 5 years old. Following a recent Photoshop upgrade, I'm finding I'm having problems with the 4Gb RAM constraint imposed by the motherboard and also the OEM Windows 7 Pro 32 bit. So I'm planning to replace the motherboard/cpu and stick in 8Gb RAM, keeping the original case and drives. I'll obviously have to use a 64 bit version of Windows. I've read that my Windows 7 Pro 32 bit OEM key will work with the 64 bit version if I do a fresh install but I'd like to know if the motherboard/cpu/ram upgrade will complicate things. Does it matter to Windows 7 that the hardware has been changed? Thanks in advance.  
PC upgrade/Windows 7 OEM key