More questions ...
Question - are you using the native kernel Nforce ide driver or the Nvidia 1.0-0261 driver for ide?
Additionally, If you run /sbin/hdparm /dev/hdd (as root) does the result indicate that you are using dma? If so, then run /sbin/hdparm -i /dev/hdd and check to see that the highest UDMA has a "star" indicating that it is being used.
In the earlier 2.4.20, I had to pass parms to the kernel to use dma with the Nforce2 chipset.