nope cant be done the mobo is specific to dell ( they made it) so they is no other replacement board available unless you get a new e521 board which you can probably pick up on ebay for cheap.
my advice in future dont buy a dell use companies that build a pc from parts ie scan 3xs if your in the uk origin do some nice pc's for the us.
i'm from the uk so i dont know what your situation is.
long story short a prebuild system from scan is perfect if anything breaks because its so easy to replace parts