There is one possibility. The FX 6 series gets you most of the game performance of the 8 series. In addition, it's possible BOTH the 6 and 8 are insufficient for certain games. Therefore, you would get no gain in performance by upgrading the videocard since you are already CPU limited.
This all depends on the games of course. Ideally, you would have upgraded ONE component, then tested the computer. If it runs fine, then upgrade the second component and test again. By upgrading both at once, you make it more difficult to troubleshoot the issue.