The only GeForce GFX Card you should really get if your serious about gaming is the GTX, the GT and GS are pants.
However the GTXs are more expensive, so if you can't afford them, then go with the GT if you really must, but stay away from the GS
Don't have any opinion on the Radeons as i've never used them before
I currently use the GeForce GTX 260, which i got for £99 something or other lol (However i ordered this along with a ton of other parts when i built my PC, so i got quite a bit of discount), but i'm sure you shouldn't have any trouble finding it close to that price, but i could be wrong :P
Anyway, i haven't had any problems with it since i've had it, and every game i play can be run in full graphics without any sort of lag (e.g. Warhammer and Aion Online i can run full graphics in all sorts of situations, i.e. mass loads of people, mobs etc)