As far as comparing what you have look at this way.
You have a 7600GS so for example a 9600GT or GSO will be around 2x as fast than you current card and you can get away with pushing a little more AA. Now if you want to push almost max AA then the 9800GT will be even better than either of those cards. A 9800GT will be around 4x as fast as your current card.
Now let's add Radeon 4850 into the mix. That one will allow you go push max AA just like the 9800GT but the framerates will be even faster. BTW I forgot to ask what size is your monitor and what res will you be running the game at? That is really the most important question you have to ask yourself when buying a new video card.