I currently have a Diamond Viper TNT2 Ultra V770 for 2D and a voodoo2 for my 3D. Im thinking about gettin a Geforce4. If i bought the cheapest G4 which is about 120$, would it have better 2D quality than my Viper? I already know it should be better 3D than my voodoo2

Of course it will be better. Miles better! We are talking here about three henerations of gfx cards in between GF4 and TNT2.
I don't really understand though why you use a Voodoo2 for 3D since your viper is way faster than V2.

Keep in mind the GeForce4 MX is more accurately an improved GeForce2 MX than it is a slowed-down GeForce4.

If you're wanting to play the 3D games that take advantage of graphics cards which run DirectX 8 in hardware, I'd consider getting the GeForce3 Ti200 which runs at about the price you mentioned.

If you want a card that will run DX 7 games faster with FSAA, than the GeForce4 MX is probably the way to go.

I'd go with the GeForce3 Ti200, though.

For more info go to the GeForce4 MX thread, and please reply there if it regards to the GeForce4 MX naming issue.

Yup. The GF4MX's DO NOT have DirectX 8 hardware capabilities, so keep that in mind if you are playing newer games.