Lets get a few things straight.
The GT 120 is the same as the 9500 GT. It is essentially the same card, with a different name and prettier looking box, that is the only difference.
The G100 is even weaker, but you don't have to worry about this since it's not actually sold anywhere.
In a nutshell:
G100 = 9400 GT
GT 120 = 9500GT
GT 130 = 9600GSO = 8800GS
GT 140 = 9600GT
The GT320, GT 330 and GT 340 are all newer iterations of their "predecessors", the GT220, 230 and 240.
GT220 = slightly faster than 9600 GS
GT230 = slightly faster than 9600 GT
GT240 = the same as a 9800 GT
That said, anything past the GT120 is better than the 9500 GT.
I don't know much about the 300 series yet, but all i can say is that they are not worth your time. None of it is -- the 9600 GT and 9800 GT are okay, but don't bother with the GT200 and GT300 series.
If you are considering on getting a sub $100 card to play the latest games, I'd say skip all of these options and go for a Radeon 4670. It's cheap, and power efficient, and best bang for buck.
Your next option is a Radeon 5770, something I would look into if you can afford to spend $140.