Nvidia is to GPUs what AMD is to CPUs
One thing that you can bank on is that nVidia will consistently come out with new designs for their graphics processors that are of great quality, and ATI will come out with processors that are very easy to mass produce. In this way the companies are a lot like Intel and AMD. AMD arguably has better designed chips than Intel. However, Intel focuses so much on their ability to mass produce their chips, and they are so good at it, their profit margins are often much better.
ATI's recent cards, their x1000 series are really technically no different than their current x850 series, however due to improvements in their fabrication processes, they have been able to clock the GPU at about 600 Mhz for their mainstream card, with their top of the line cards clocked even higher. They have their memory operating now at about 1 Ghz.
Nvidia has double the pipelines on their card and can therefore perform better at lower clock speeds. This means of course better power consumption characteristics as well as possibly more headroom. Still, as the battle between AMD and Intel has shown us, with their shallow instruction pipelines AMD ran into clock speed issues earler than Intel. Netburst may be dead, but that hasn't stopped the video card manufacturers from locking horns in a race for the first 1 Ghz GPU. It will be interesting to see who gets there first, my money is on ATI, but GPU speed doesn't always translate into performance. ATI's new card, albeit with less effort, puts them just at nVidia's level again. Look for nVidia to leap forward again with new innovation soon.