If you're intended final output is SD NTSC TV, then it would benefit you to get a SD monitor, you can preview HD pixel for pixel on teh Apple, but you still need a working montior if you go that route, plus Apple Desktop display isn't quite as accurate as using an output card/monitor setup.
The Cinema displays are rated for HD monitoring. You do need an external monitor for SD. I just run through an old GL-1 via firewire for SD. But HD can be done with the Apple Cinema Displays just fine.
It will output NTSCSD, so I would assume it can output DVCPRO50, but you will only get DV quality since the outputs on the GL1 is composite. I could be wrong, so someone correct me if so. If you get a Kona card or a Blackmagic card then you shouldn't have any troubles outputting to a monitor, plus most of their cards offer hardware acceleration as well as other features. I've been looking at the JVC TM-H150, but lately I've been thinking about spending the extra cash on something with SMPTE-C phosphors for higher color accuracy.
I've heard good things about the JVC monitor, you can send your outputs of your GL1 to the JVC. I saw a used Panasonic with SMPTE-C phosphors for aroun $800 the other day. I'm not 100% sure how good the SMPTE-C's are, but from what I've read they are highly accurate. Plus, I'm still going back and forth on what type of monitor, HD or SD, will suit my needs best right now.
Under your sequence tab there is a button that reads "RT" (for "real time", I believe). if you click on that it should give you the option to view through your firewire connection. And it should show whichever window you are playing.
The only problem I have had with this is when I start FC with my FW device off. If I turn it on after FC starts, the FW option is greyed out and I have to restart the program.