You should also check the website for your current MOBO manufacturer. Mine is only a relatively cheap one (a bit like the computer), the whole pc came from one of the pc chain stores with their badge, but the pc it's self was manufactured by Medion (German) though the mobo is actually made by MSI (Taiwanese).
I had to check because I wanted to add a "proper" soundcard (the onboard one is a bit of a pain to get working) and network card - on the face of it, it should have had 3 pci slots, when I opened it up to check, I couldn't quite work things out, so I did more digging at the mobo site, and it turned out that what I thought was the third pci was actually a modem riser slot - i.e. different from a pci and so I've only got 2 - my mobo is the even cheaper version than I thought it was, though I did manage to get the pdf mobo documentation for it.
Could this be the case with yours, as modem risers and pci look similar, but as far as I can work out, they are incompatible ???
just a thought.