GF4MX420 is roughly = to GF2GTS
GF4MX440 " = GF2Ultra
GF4MX440-8X is faster still
GF4MX460 is a bit faster again

Even so it is still the GF2 core at the heart albeit with a speed boost, enhanced image quality, dual display, TVout, DVD playback etc. Even the GF4MX460 is still slower than a stock GF3TI200 so really you are best to go for one of the following cards ...
Rad8500LE-128MB (or other Rad8500 derivitive) - I wouldn't consider anything less inc the Rad9000PRO
GF4TI4200 - Great value even at stock speeds and plenty of room if you do want to o/c
Rad9500PRO - Great value if you can afford the price tag

Even the Rad8500LE will be noticably faster and more capable than a GF2GTS but if you can afford the extra go for the GF4TI or Rad9500PRO, all three cards are IMHO 'great value for money' so make your choice and take the plunge! Of course do ensure your CPU is decently fast, given the price of XP2000+ type CPUs it makes sense to avoid choking your new gfx card.