OK, so let's say that this is appealed to the First Circuit Court of Appeals, and the judgement is upheld. The first ciruit includes the District of Rhode Island. In Rhode Island, marriage is held to be the union of one man and one woman (for the moment. Legislation has been introduced to allow gay marriage, to ban gay marriage, and to introduce civil unions). After the ruling by the circuit court, though, that can't be the case, because such a definition violates the equal protection clause.
So, the effect is to declare that DOMA is unconstitutional because the Federal government can't tell the states what to do when it comes to marriage, and it would simultaneously use federal power to overturn a state law defining marriage.