It should fit easily, the only thing that you will need to do is remove the protecting thingy from the slot under the current one (at the same time it'll cover the PCIe slot under the second slot). If the top fans are 12cm ones then you should also have enough space horizontally, as it's only 20cm long (give or take).
It would be awesome to know what case you have though, so exact dimensions can be checked, just in case (though I doubt it could be too small).