There is not alot of difference between the mx420 and the 440, the latter has a slightly faster core speed, and 128bit memory bus whilst the former has a 64bit bus (going back a fair bit in technolgy there) the 420 has 2.7GB/s of mem b/w compared to the 440 which has either 6.4 or 8 GB/s (depending on if its agp4x or 8x) High end cards these days are in the region of 30+GB/s
http://www.nvidia.com/page/geforce4mx.html has all the specs on the 4mx, note no pixel or vertex shaders, as quite rightly said its a modern (aka cheap) geforce 2 gts. Solid performer really on older games, nothing fancy but will be 'eck of alot faster than the sis card.
Technology comparison with the rest of the '4 series
http://www.nvidia.com/page/geforce4.html
nfiniteFX (II) is there name for pixel and vertex shaders, ie dx8+ compatibility