If you have the binary nvidia drivers and libvdpau, then rebuild MPlayer. MPlayer will detect that have vdpau support available, and you'll be able to use vdpau for hardware-accelerated decoding!
There are several SlackBuilds you can use, including one included with Slackware's sources. To also get support for playing commercial DVDs, run it with:
Code:
USE_PATENTS=YES ./MPlayer.SlackBuild
Edit the SlackBuild script and change the VERSION variable at the beginning to the current date, and it will pull and build latest version of MPlayer. The current versions of MPlayer feature multi-threaded decoding (details are on mplayerhq.hu), so this is recommended. (You'll also have to remove the obsolete --enable-large-files switch).
BTW, while none of the above will hurt you, you can really get by with doing nothing and just using version of MPlayer included with Slackware and/or the VLC package Samael linked to. I have a very similar setup to yours (Athlon II X3, Geforce 210), and both can play 1080p trailers from apple.com just fine.