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.