It is possible in multimonitor?
 

Using a nvidia graphic card with multi monitor with two monitors. Can I have one with xbmc and other with normal KDE (firefox, xchat, ...).

Can I do this in linux (Slackware)?

Yes you can AFAIK
 

I don,t have multiple monitors so I,m guessing but I am a really good guesser...
In xfce its settings > system settings > display > multiple monitors so if you are using KDE or something else you should find what you are looking for.

yes, you can do this

just run two X sessions, one on each monitor (use nvidia-settings as root, to set that up), and in the main one's autostart file, start xbmc on the second display

e.g.
DISPLAY=":01.1" xbmc&


note that if you use KDE though, you need to disable multihead, else KDE will take over the second monitor automatically
the file is:
~/.kde4/share/config/kcmdisplayrc

and under the section labelled as "[X11]"
add the following:
disableMultihead=true

How can I use xfce on one monitor and kde in another.

EDIT: I think it is possible.