Not on an 8400, but on an XPS G4 with 3GB RAM - which is just a fancier version of the 8400 from the XPS line of gaming computers.  Windows 7 installed with no additional drivers needed and has run like a dream for almost a year now.  The only thing is the game port on the Audigy Z sound card is not supported by Windows 7 (but the sound card itself works fine), which was not a big deal for me.  Since W7 will eat a GB just to stay alive, 2GB might be enough if you aren't doing any really memory-intensive stuff like hard-core gaming, video-editing, modeling, etc.  My kids play Age of Mythology all the time and my wife is a photographer - she is its primary user, and rarely does she hit a wall at 3GB.
Hello Has anyone had any experience with installing Windows 7 into an older Dell Dimension 8400 with 2 Gigs of Ram? Will it work? Thanx Mike    
Dell Dimension 8400 and Windows 7?