i think that the motherboard you have now is terrible, knowing what dell probably shipped it with. so yea, you should upgrade your motherboard.... the matter if it fits or not, um i think it would fit fine, but you should definitely call dell or look into that.
You're motherboard is of standard ATX configuration ( other than the 8x only PCIE slots ) and is replaceable with any other ATX board of the same size.
You really should hit up the Dell forums, they have sections there dedicated to the XPS630. Another forum option would be My630i.com