The X1900 has 16 pixel rendering pipelines, with 3 pixel shaders each. This means it has 16 pipes for texturing/fillrate purposes, but each pipeline has 3-pixel shaders amounting to a whopping 48 pixel shaders total.
It's a totally different architecture as it's more forward-thinking that cards with 16-24 texturing pipelines may not ever achieve their fillrate potential due to larger/more complex shaders attributing to in-game visuals vs. just normal bitmap/image textures. Games such as Oblivion prove this is where future games are going.. as has every major developer also concurred.
So while the 24-pixel/texture pipeline model of NVidia's cards offers much more fillrate (for normal texturing), the X1900's offer substantial more pixel shader power for running shaders. If you create a hypothetical model where, say, 70% of on-screen graphics are the results of shaders instead of just flat textures.. you could easily see where having insufficient shader horsepower might lead to one's fillrate potential to never be reached... and thus better performance for cards with less texturing muscle, but more PS muscle.
The X1600 Pro is another good example of this. It's only got FOUR(4) texturing pipelines, but again 3-pixel shaders per pipeline (total of 12 pixel shaders, 4 pixel/texturing pipelines). It's able to easily beat or hold it's own against 8 pipeline cards in games like Oblivion or other shader-rich newer games... but it get's it's **** handed to it in games that use heavy, heavy texturing (mainly older games) without a lot of shaders.
Hope this helps!