I also just got a shiny new Dell 8400 , built on the Intel 925X chipset (i.e. Intel ICH6-R aka "Soft RAID" controller) but I ran into the same problem before when trying to boot Knoppix on a Dell Optiplex GX280 (live at a customer, nonetheless.... Basically the only way to have Knoppix booting on that machine is to get into BIOS and put the controller into "compatibilty mode" (or whatever your BIOS calls the PATA, backward-compatible mode). Play with it / install it, download the relevant SATA driver (in my case it's the "ata_piix" driver IIRC, YMMV) compile it as a module, revert to SATA-native mode in BIOS and reboot.

See also this thread (also containing pointers to the webpage that summarizes the situation of SATA support in Linux kernel -- currently shoddy at best).

http://www.knoppix.net/forum/viewtopic.php?t=13273

Hope this helps....and that Google will index this page to help other poor souls out there