I personally wouldn't call the PS3 web browser a capable browser in this day and age. Even from launch it was years behind other browsers. I tried to stream stuff using it, but it was just so slow (and it's not the connection; that I'm sure of).
I'm fairly certain there's no way to install XMBC onto it. Depending on your video formats (I'm sure you want the apps XMBC has to offer, too), you probably don't need it for playback purposes for the ps3. The only problem format I've run into is the .mkv container.
Why else do you want a ps3? Games? Blu Ray? If you just want an XMBC box, I'd recommend buying and hacking an ATV2, since it's much cheaper and can run the software you want. If you want the games and blu ray, the ps3 is still a very capable device for video playback. I use Rivet (free courtesy of the Little App Factory) to stream local content to my ps3 and it's very seamless. The biggest limitation the ps3 has, in regards to playing back files, is that it can't handle files larger than 4GB (internally; streaming doesn't seem to be a problem) and has trouble with BD+ rips (copy-protection-related).