I'm not sure if you can replace motherboards on pre-made systems such as Dell, HP, Gateway.
I read somewhere that they use custom motherboards, so introducing a new motherboard it might not;
a. Line up with the available mounting holes
b. Be able to connect to the peripherals in the front of the case (reset, power, lights)
So I'd do a little research and try to find a manual online (if you don't have one) for your specific system.