Yes, you're a bit off the mark with inittab, but fret not!
How do you start X normally? Through a display manager like GDM or KDM (graphical login) or console login (no fancy graphics)?
In either case you can create an .xinitrc file in your home directory if it doesn't exist already. This file is read for startup commands when you run startx from the console. A similar file, .xsession, is read when you select "Custom session" in GDM and I would think there is a similar entry in KDM. Here's mine:
fbpanel -p pager &
aterm -geometry 80x24+0+26 &
xrootconsole -fn lime -fg gray /var/log/messages -geometry 166x8+180+674 &
The commands with an appended ampersand (the "&" character) will start and fork to background so the commands after them will be executed too. The final command, "exec openbox", launches my window manager. The exec prefix means the script is replaced by the openbox process. This means that when openbox exits, the whole session will end (logout if I use GDM or back to console if I don't).
Since .xinitrc and .xsession are basically the same I only keep an .xsession file and then have a symbolic link called .xinitrc pointing to my .xsession file.
When I last ran KDE (ehh, back in the 1.0 days) there was a "startkde" script. There is probaby a similar one now, perhaps with the same name. So a very simple .xinitrc/.xsession to start KDE would be:
Hope that helps!