I would upgrade to the HD 4830, HD 4850, or HD 4770 to make the money you spend count.
The HD 4770 cost the most, but also consumes the least amount of power of the three, thus you may save money on a new PSU if the current one is weak.
The HD 4850 is the most powerful GPU of the three, while the HD 4830 is the weakest.
Power consumption:
HD 4770 => 50w - 55w
HD 4830 => 85w
HD 4850 => 110w