The Radeon 4850 is faster in just about every game and setting over the 9600GT. Just take a look at the charts here on toms hardware and compare the two. The 4850 also has built in audio over HDMI while the 9600GT needs to connect to your sound card with an SPDIF pass through cable to accomplish the same. The 9600GT uses less power though and supports CUDA for things like Physx. Of course, it's not really fast enough to play a game at high settings with Physx enabled
When the 9600Gt was introduced it was a midrange card. Today it is considered a minimal gaming card. The HD4850 was introduced as a low high range card, it is still considered a good midrange card it is competing with cards like 9800GTX+ and GTS250 in performance.