This could be an interesting year as people upgrade from the venerable 8800 series which is the most popular graphic card series tracked on Steam. In terms of market share for discrete graphics card, nVidia has the type of dominance not too dissimilar Intel for CPUs. As of the Feb 2010 survey, nVidia cards are used by 61.05% of Steam users, while AMD/ATI by 29.82%.
It's not reasonable to expect significant change, but it will be interesting to see if Steam's survey is consistent with general opinion on Hexus by observing the growth of new cards.
Jan 2010
Feb 2010
Mar 2010
April 2010 (all)
April 2010 (DX11)
May 2010 (N:61.88%, A:30.92%)
May 2010 (all)
May 2010 (DX11)
June 2010 (N:59.11%, A:32.98%)
June 2010 (P1)
June 2010 (P2)
September 2010 (N:59:11%, A:32.98% - Okay it's obvious that they don't update those figures monthly)