Yeh no PCI-X on that mobo... i assume you already have a PCI-x controller card and that is why you want that slot. PCI-x is hard to find these days unless you goto a server board (lotsa money). The only deskop board i know of off the top of my head is ASUS P5K WS which has 1 PCI-x 1 PCI-e16x and a couple of 1x. http://usa.asus.com/products.aspx?l1=3&l2=11&l3=534&l4=0&model=1691&modelmenu=2
Not sure how available these are anymore.
Your other opinion would be to buy a new high end raid card that supports SAS and SATA drives . My Adaptec 3805 supports both and you can have 4 SAS and 4 SATA is you want.
And yes other cards will work perfectly fine in PCI-e slots. doesnt have to be GPU
most boards that have pci-x and pci-e are workstation boards and high $. I would not bother with pci-x as it is quickly fading to pci-e. PCI-E card can be used for the SAS controllers. I have had an Adaptec 31605 SAS controller on the PCI-E boards of 3 different boards.