does slackware 12 can best run in P-III 500Mhz with 128 MB?
and also...
can it be join in a Domain?

Runs "best" on P3? I didn't get that question. Anyway, the answer is it runs on a p3.

Can it be join in a Domain?

Yes. Samba can join any distro to a domain.

My slack runs on p3 128mb ram.
I can run KDE but I recommend blackbox or xfce.
don't use xine or amarok, use command line multimedia player such as madplay, mpg123, sox, or mplayer. It runs best.
Turn off all services that u dont need such as mysql, php, or httpd. Use fast filesystem such as reiserfs(I'm not sure but I heard that reiser is faster than ext3, btw I'm using ext3).

Hi,

Yes!
Yes!

You should read the Slackware 12.0 CHANGES_AND_HINTS.TXT, UPGRADE.TXT and RELEASE_NOTES?

If you are going to try Slackware® then I would suggest that you read the 'Slackware® Essentials', the official guide.

These links and others can be found in the 'Slackware-Links' .

I run Slackware on a IBM TP 600E...that's a 400MHz PII! It runs fine with Xfce (KDE is slightly slow, but still very usable). I did upgrade my memory to 288MB which does help.

As previously mentioned don't run services you don't need. Speaking of that, unless you are tied to KDE or any of its apps don't install KDE at all. Either that or make sure you don't load KDE services until you need them.

If you do plan on using a command line music player I suggest you look at herrie instead of the others mentioned. There is a slackBuild for it at slackbuilds.org.

I don't think reiserfs will add a significant boost in speed, but using ext2 (not 3) might help some. ext3 should be fine. Just don't let the drive get too full or you will get some fragmentation (this is true for the other filesystems as well).

If you are new to Slackware you might also want to check out VectorLinux. It's a spinoff of Slackware that works really well with older hardware. It has some admin tools to help you as well.

Finally, if you want to get the best performance out of your machine build a custom kernel that has only what you need.