After looking up your Dell I can not find anywhere that you have a PCIe x 16 slot. You might be able to use a AGP type card. But I don't know for sure. It does say you have a PCI slot 3 of them.
But that is not a PCIe slot. So unless you have put in a new mother board (MOBO) then I really doubt you are going to get what you want. I have posted your PC so you can see all the info.
I also linked you to the AGP GPU's. You have to make sure that you can even upgrade the GPU. I would recommend call dell and asking if you can and what type GPU you can use. Good luck to you.http://www.pcworld.com/product/14100/dell_optiplex_gx260.htmlhttp://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&N=-1&isNodeId=1&Description=agp+graphics+card