nForce2 (if later revision board at least) will give you upgradability by supporting the new 400MHz fsb Barton chips
KT400 = no 400 MHz chip support, KT600 will give that support in the near future.
nForce2 - better memory management
KT400 - slightly behind the times, poor memory management and sometimes AGP 8x stability issues (but I think those are all cleared up via bios revisions)
I personally own an Asus A7N8X Deluxe and love it. The sound quality is superb. The dual NIC comes in handy for LAN parties when I need to give up my switch for others to connect, then the WAP can stay online by bridging the two NICs on-board.
The SATA is turned off, but is available. 1394 firewire and 6 USB ports ready to use. AGP 8x and it works awesome with my Asus GF4 Ti4200 128MB AGP 8x card. Granted, no performance increases with AGP 8x but it works without issue. 3DMark registers it as an AGP 8x card.
Anything else?
