You do realize that there are dozens of Dell Optiplex models, right?
We need a lot better description of what you have now if you want us to provide advise that is worth anything to you.
Without knowing anything about your internals, the best I can tell you is that it would probably be fine if you got a HD 6850 and an XFX 550w to go along with it OR if you have a bigger budget maybe a HD 6950 and an XFX 650w instead.
Also, DELL doesn't make gaming cases, all their cases are bottom of the barrel for gaming usage. They have little airflow and all the ones I have ever seen had top mount PSUs which is bad.
I would suggest you consider replacing the DELL case with an Antec 300 or HAF 912 instead. Those are both low end cases made for serious gamers with good airflow options and bottom mount PSUs.
I have tried to convert some of these does not work to well unless you have top of the line opti. The PCI-E slots only provide 25watts in most dell motherboards not the PCIE complient 75watt this makes any high end video card not work. I tried and tried to stick a 240GT into and new opti with a i5 2500 and no go it would beep fans would come on but no screen. I did get a 9600gt to work but a i5 2500 with a 9600gt odd this worked as 240 should use less power I think the reason was the 9600 had external power conector and the 240 did not so it needed the 75 watts from PCI slot. Anyway I looked up the specs on the Desll Spec PDF and it lists 25 watt on x16 PCIE so nothing can be done. This limits what you can do with these desktops but i guess they want this as there is low air flow in these cases.