I updated my SUSE with YOU, that should have installed their new kernel, the 2.3.21-144.

But when I type uname -r, it says 2.4.21-99-athlon!

Then I checked the /usr/src and there was the new version: /usr/src/linux-2.4.21-144 and
/usr/src/linux-2.4.21-144


My question now is, how to activate this newer kernel instead of using the old one!

I'm rather much of a n00b to Linux, but isn't 2.3 older than 2.4, or am I missing some important method of naming kernel versions here?

For a binary kernel, the files should appear under /boot.
Files under /usr/src are only for compiling it by hand.

So, if you want to use a new kernel, you must see if it exists under /boot and them instruct your boot loader about its existence.

I.e.:
I add kernel 2.6.0-test11, so, under /boot appear these files:

/boot/config-2.6.0-test11
/boot/System.map-2.6.0-test11
/boot/vmlinuz-2.6.0-test11

and so I add an entry into my bootloader config file (/etc/lilo.conf)

image=/boot/vmlinuz-2.6.0-test11
label=260-test11
root=/dev/hda1
read-only
optional

and finally I run 'lilo'

Chances are that you are using the grub bootloader, which I don't use but I guess there's a config file under /boot/grub

thank you very much for this!