If you want to stream mpeg 1-4 or DivX content from your PC to ps3, run TVersity, Playstation Media Server, or any other DLNA media server on your PC... you can get the above software for free.
A little bird told me you can use Windows Media Player 11 to stream DLNA, but I haven't tried it - I am happy with TVersity.
I have also heard that you can stream VOB files directly. I haven't confirmed this.
I have also heard that the ps3 artificially imposes a 2GB limit on the aforementioned files for copy protection reasons - I haven't confirmed this either, but I've already encoded most of my movie collection to under 1GB - (because I encoded non 720p stuff, oldschool before I had HD)