Hi,
Either of the cards you have selected will be quite a step up from the 9600, I just changed out a 9800 to a 7600GS in my Son's system and it runs alot better, he mainly plays Warcraft etc but he is happy with it.
I used the Gigabyte one, it is Silent Pipe, no fan, but requires good air flow in the case to remain cool.
IMO 512Mb on the 7600GS would be pointless as 7600's should be running a 128bit memory and I don't think it would be fast enough to be able to use it properly.
I read somewhere about some manufacturers making the 7600GS with only a 64bit memory, the performance of such a card would be horrible.
I can only recommend you read the spec sheets on any card you look at very carefully before you choose one.
Good luck and Merry Christmas.