nVidia is an effective company, they flooded the market and pushed their budget chips into systems everywhere, it amazes me to see on occasion the new systems that still "boast" about the TNT/TNT2 power graphics they have, now being pretty much replaced by the GeForce 2 MX of which that is now being taken over by the GeForce 4 MX. Ti 4200 has little to do with anything, its still a high end card that doesn't sell like the MX hotcakes, consumers are largely uneducated and dont mind "getting by" on their GF MX performance, they don't expect anything more, if they did they'd get a 4600, 9700 or better.
For us educated computer enthusiasts, we are an obviously biased population and most of us will have the Ti 4200s and 8500/9000s of the video card market giving the illusion that nVidia is selling mostly 4200s but the money truly is in the budget market. But to gain dominance in the budget market you really need to gain a dominance in the enthusiast. People by GF MX cards not because they know they are fast (they aren't) but because they have nVidia written all over it and they know nVidia makes fast cards (even if the fast cards they make don't include the ones they just bought, they don't know that).
Oh and btw, nVidia can't claim the top spot, they've always had it for a long time. Starting to see a lot of budget and high end ATI cards in stores, however. ATI's establishment of domination in the high end for the past few months has certainly gained them favorable reputation. All they need to do is exploit that in the lower segments of the market, get people to buy the name that produced the mighty 9700/9800...