Originally Posted by ssc456
i would say no.
thats an opinion though the reason i say is that dell motherboards are an awkward shape
I have no idea where you get that they have an awkward shape, they are either ATX or mATX most of the time and are the standard rectangle shape, if you're trying to say that they're sometimes proprietary, then that's a whole different matter and has nothing to do with shape...
I would say that they look about the same size except the 8400 has a PCIe slot, but I would google the motherboard specs for both just to be safe if I were you, my guess would be that they're both mATX (micro ATX) form factor and should be swappable...