Stock Slackware 13.0 only supports the Intel 82865G in vesa mode.
To get DRI requires updating the kernel to 220.127.116.11 or later. Upgrading the Intel driver from 2.8.0 is also worth considering.
There is also another thread with information on this.
Using the upgrades, I get a usable desktop with desktop effects on my server, but performance is not stunning.
In my experience, nVidia graphics are a better option, but require the proprietary driver for 3D effects. Installation of the proprietary driver is usually trivial using the supplied install script if you have the kernel source installed.
You do have to rebuild the kernel module after a kernel update. If you have changed /etc/inittab to boot directly to run level 4 (unlikely on a server), make sure that you change back to booting to run level 3 before rebooting with the new kernel so that you can reinstall the driver. Occasionally a driver update is required so that it will work with a later kernel version.
You can also install the driver and kernel module using Slackbuild scripts, but you need to know your xorg.conf settings. It is probably easiest to install the proprietary driver directly, letting it update your xorg.conf. Then you can uninstall the driver, build using Slackbuild scripts and install the packages.
Another problem with adding another graphics card on some hardware is that even if the onboard graphics is turned off in the BIOS, the chipset may still be detected. Setting a BusID option in xorg.conf to point to the nVidia card should fix that.