Chris is correct...you can burn a blu-ray with Roxio Toast. However you won't be able to play it back on your mac.
Another workflow is using compressor/Adobe Encore CS4. It is more time consuming and I haven't done it myself but I hear you get better results that way. Encore is a full featured authoring program like DVDSP, so you'd be able to do more with it than with Toast, like create full menus and so forth, but if you just want a one-step disc for immediate playback then toast would probably do the trick just fine.
As for the burner itself...you can get an internal one for around $200 that should be compatible with your system. I always use newegg.com. I'd check there first.
Best of luck.
John Baldino
GoboMedia LLC
http://www.gobomedia.net (under construction)
"Philosophy is questioning without answers; religion is answers without questioning." - Anonymous