The GF2 Ti is faster, the MX420 typically performs relatively similar to the GF2 GTS-V... which is to say somewhat faster then the classic GF DDR, but slower the the GF2 GTS.
The GeForce2 Ti will solidly outperform it in games.
If you can afford a bit more you may want to take a look at the Radeon 7500 or GeForce4 MX440 though.
The GeForce3 Ti200 and especially the Radeon 8500LE would both be considerably better... though their typically a good 35% more costly then the MX420/GF2 Ti.