In preparation for the launch of World of Warcraft: Cataclysm, we have updated our recommended minimum system requirements. These new requirements correspond with patch 4.0.1. We suggest players check out our related support article for all of the details to ensure your gameplay experience is as ideal as possible.

http://us.blizzard.com/support/article.xml?locale=en_US&articleId=21054

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The unsupported-but-maybe-still-possible section
Basic information can be found here. Things may be slow and some objects may look odd. We can't help you run the game on these so you're on your own. This is just a courtesy section.

Intel GMA 865s - seem to be able to run the game with the 14.17 XP drivers. http://downloadcenter.intel.com/Detail_Desc.aspx?agr=Y&DwnldID=9506&ProdId=1044&lang=eng
These cards do not work on Windows Vista or 7.

Intel GMA 915 - seems to run the game ok with the 14.24 XP drivers. http://downloadcenter.intel.com/Detail_Desc.aspx?agr=Y&DwnldID=14493&ProdId=1764&lang=eng
14.25.50.4764 seems to have an issue where moving the mouse freezes the game screen until the mouse stops moving.

Intel GMA 965/G31/G33/X3100 - seems to run the game ok with the latest drivers and in fixedfunction or swtnl mode. Some people are reporting that older drivers may have trouble alt+tabbing in and out of the game if it's set to full screen mode

Via Unichrome - People are reporting that turning off sunshafts can help with the strange lines that are rendered at the login screen. Fixedfunction can also help.


The not-possible section
Apple PowerPC based Macs - no codepath for it
Athlon Thunderbird-class processors - we now require SSE support