NOTE: I'm not in front of my PS2, so some of this might be spelled otherwise.
You can safely leave it to none, as all modern european TV-sets support both, PAL and NTSC. However, as most developers doesn't really bother to fully port their games to PAL markets you might have "black bars" at the top and bottom of the screen (pseudo wide-screen, flattened, thou), you might find "force NTSC" more enjoyable.
I've no experience with HDTV, yet.
"Only those who try will become." -- Kimahri Ronso