You may try doing a BIOS flash, that'll usually do the trick. But if I remember correctly (and I probably don't) the NF7-S doesn't support 400MHz FSB which would explain why it's not detecting the 400MHz FSB Athlon XP 3000. I think the NF7-S2 does support 400MHz FSB.
[EDIT]Wait, never mind. The 3000 is 333MHz FSB. I was thinking of the 3200 @ 400MHz FSB, sorry about that. So yeah, try the BIOS flash.[/EDIT]