1. My setup
I have a mac mini wired to my D-Link 655 router. My PS3 is hard wired to the same router. The router is nice since it enables jumbo frames (1000 MB/S) so stuttering is kept at a minimum when streaming. PM me if you have this same router and want to go into more details.
2. General Settings
I would set a static IP for the PS3 and the computer from which you will be streaming. Also, enable the "Media Server Connection" option on the PS3 under the Network Settings option. And make sure Upnp is enabled as well.
3. Download the following file
http://ps3mediaserver.blogspot.com/
Download it and install it in the computer from which you will be streaming from. Open the program and go to the “navigation/share” settings tab. Under shared folders, add the folder where the media is currently saved. Save the settings and restart the program. You will know the program is working correctly if the “status” tab says "PS3 has been found". If it does not say this, I would refer to the PS3MediaServer forum. Its a very useful area and the developer is quite active on the site.
4. Play the Movie
After you restart the program, I suggest you reboot the PS3 as well. After the reboot, you should see the PS3 Media Server show up as a directory under the video section of the PS3 browser. Access the directory through the PS3 browser and select the media file you would like to play. Assuming everything is working correctly, the movie should start to play. Enjoy.