I'd suggest the AMD X2 4200+ with a 7900GT would be better for gaming
The graphics card matters a whole lot more than the processor nowadays, and even though there are such things as bottle necks whent he CPU can't catch up with the card, the 4200+ won't have this problem.
But, if your doing other things as well like maybe video editing, then a better processor will help better, so the second would be the best option...
That said, I don't know too much about that Conroe CPU, but its supposedly better than the X2 AMD line
But for gaming, if thats the most you'll be doing, the 7900GT will do VERY nicely, but the 7600GT is also a very good card, and you'll get a better CPU out of it for other tasks, but the 7600GT in no way can run new games at full settings and resoloutions, yet, the 7900GT could.