The MX420 is the black sheep of the GF4 MX family. Actually, IMHO, within the entire GF4 MX family, only the MX440 is worth considering, since its in a unique price point with no other direct competitors. The MX420 is simply too slow, hampered by its slow SDR memory, and will be outperformed by even a lowly GF2 GTS card. The MX460, while faster than the GF2 Ultra in most cases and even the GF3 Ti200 in some very rare cases, is all in all an inferior card to the GF3 Ti200 yet costs more than comparable AND better performing/featured cards like the GF3 Ti200, Radeon 8500 and Radeon 8500LE. It also approaches the price of the GF4 Ti4200.
So, if you're strapped on a $50 budget, get a GF2 GTS/Pro/Ti/Ti200/Ultra/V/whatever, as long as its not a GF2MX, but if you have a bit more to spend, say $70-80, get the GF4 MX440. If you have $100 to spend, get either than GF3 Ti200, or better yet the Radeon 8500 or 8500LE. The GF4 Ti4200 might be too much for your budget.