You would get a nice performance bump by going to HD5770 if your PSU is up to it.
The HD5770 actually only uses less than 10w more than the standard version of the 9600GT during normal usage. So if he can handle one he can very likely handle the other.
@OP: here are charts including the 9600GT that shows its performance relative to most current cards at various resolutions;http://www.techpowerup.com/reviews/Axle/Radeon_HD_5670/29.html
If you are going to upgrade I would try to get a DX11 card at this point which means either the HD5750 or HD5770. What is the native resolution of your monitor and how much are you willing to spend?