After having problems with NVidia installation, finally i can solve the problem and also i managed to run compiz on my Slackware system. Few days ago, i couldn't install any NVidia driver due to mismatch kernel headers (probably it's because i installed kernel-headers-18.104.22.168 after i upgrade my kernel, so it overwrites the link to the current kernel-headers file). Last night i tried again and it still failed, so i tried to upgrade my kernel to the latest version (22.214.171.124) and recompile NVidia driver again for this kernel and hey... it's working again. I can run glxgears and any 3D-enabled screensavers again.
I also try to run compiz on my system last night and it worked just like charm. It's just that i don't like that it removes the Title bar on top of each application, so there's no Minimize, Maximize, and Close button on the upper right of the application. It makes the application cannot be moved also. I still also see lots of warning/error messages in the terminal. Well, i'm not surprised. Compiz is still in under heavy development and it still evolving. I guess i'll have to wait for several other release before i can get a stable and fully working compiz.
Well, at least i can now enjoy 3D Acceleration again.