I have both cards:
==GF2 GTS Ultra 64MB (DDR) in a p4 1.3 w/386mb PC 800 RDRAM
==GF4 MX420 64MB (I *think* SDR, not sure) in a p4 2.53 w/512mb DDR RAM (333MHz, if I'm not mistaken)
Both installed with latest official Nvidia driver 4109, with all settings/values at default. The GTS quite noticably outperforms the MX in Quake 3 Team Arena, g_speed increased to 480. No benchmarks necessary to see the difference. If you want numbers just enable cg_drawfps "1" and you will see the frames/second.
The MX definitely has the better 2d, much smoother, probably due to the AA and other goodie features that the GF4s have? Also, if Quake 3 were played where everybody just stands still, then the MX would be better, lol. i.e., a stationary image is simple softer, smoother, no "jagged edges". But start running around and you will notice diminished 3D performance with the GF4 (again, this is without overclocking and probably using the SDR variety of memory). If the MX is DDR then I would choose it. Otherwise, there is no question that I would choose the GF2 GTS over the MX.