2.4.20-9 != 2.4.20.20-9 ?????
 

i try to upgrade my kernel and it shows:

[root@localhost neo]# /sbin/insmod /lib/modules/2.4.20-9/kernel/drivers/net/nvnet.o
/lib/modules/2.4.20-9/kernel/drivers/net/nvnet.o: kernel-module version mismatch
/lib/modules/2.4.20-9/kernel/drivers/net/nvnet.o was compiled for kernel version 2.4.20-9
while this kernel is version 2.4.20-20.9.

anyone can explain this?

You need to update the nforce driver after you upgrade the kernel, unfortunately Nvidia doesn't provide a precompiled package for kernel 2.4.20-20.9 for Redhat, therefore you'll have to get the source(the one under Alternative installation on the download page) and compile it yourself(hint: follow the README)
Also make sure you've updated the kernel sources to 2.4.20-20.9 through up2date.

thx demonbane, now i know why:

[root@localhost neo]# rpm -qa | grep kernel
kernel-pcmcia-cs-3.1.31-13
kernel-source-2.4.20-8
kernel-utils-2.4-8.29
kernel-2.4.20-20.9
kernel-doc-2.4.20-8

and i've download the kernel-source-2.4.20.20-9, but i couldn't find the other kernel utilities newer then kernel-utils-2.4-8.29. if i don't change it, will it take any effect?

nah you don't need to match kernel-utils to the current kernel version, if there isn't an update from redhat available then you're good.