Crysis works in DirectX9, but it was marketed as a DirectX10 game.
Quite simply, if you wanted the highest graphical setting (Very High), you needed alegedly DirectX10 to have those effects. According to CryTek and MIcroSoft, those effects are only available on their spanking brand new DirectX10, and according to them DirectX9 wouldn't be able to display those at all.
A hacked demo later [http://arstechnica.com/gaming/news/2007/10/easiest-mod-ever-enables-directx-10-features-in-the-crysis-demo-on-xp.ars] showed that both Crytek and Microsoft are full of shit in every sense of the word.
This is sadly not the first [http://shinakuma.gametrailers.com/gamepad/index.php?action=viewblog&id=263660] time Crytek uses downright false claims to support a new piece of hardware to market their games.
Sure Crysis did work in Windows XP, but according to CryTek and Microsoft, DirectX10 had some magical fairy dust that made the game look and run prettier.
Also, CryTek and Intel advertised [http://www.youtube.com/watch?v=-CD-mJMB7Lw] heavily how Crysis would use Quad Core and run better on that hardware when in reality, it was just a big bag of bullshit [http://www.extremetech.com/article2/0,1558,2209093,00.asp?kc=ETRSS02129TX1K0000532]
I know this post is entirely offtopic, but I just wanted to clearify and support my claims.