iMovie is a fine non-linear editing application for beginners. But as you advance your craft, you'll quickly outgrow its limitations. It's a single-track video editor, so if you need to layer video clips and what not, you need to step up to FCE or FCP.
You don't need FCP for AVCHD. FCE is more than sufficient, unless you really want the flexibility the other bundled apps give you. In a nutshell, what makes FCP different is:
Motion = Well, a motion graphics application. FCE itself has rudimentary motion controls built-in like FCP does.
Color = Professional color grading software. You really want a 2nd monitor to use this application effectively. For basic grading, FCE has color correction plugins built-in.
DVD Studio Pro = Professional DVD authoring software. Basically, it does everything iDVD won't do. But no Blu-ray authoring support, however.
Cinema Tools = Tools for conforming projects shot on film with offline digital edits.
Outside of these bundled applications, FCP supports professional video codecs (DVCPRO, HDCAM, XDCAM, REDCODE, etc.) that FCE does not.
Only go Pro if you are a Pro, its not worth the extra money IMO if your not going to use motion, dvd studio pro. don't even bother installing color on a mbp you NEED a mac pro etc.
I would recommend using an external drive as a scratch disk w/ FW 800
You don't NEED a Mac Pro to run Color. It'll run on a iMac with an external display in a pinch. To use Color PROPERLY with a calibrated broadcast monitor that you'll probably feed with an SDI signal, a Mac Pro would certainly make things a lot easier because they can accept expansion cards. But I know a few people that run Color just fine on iMacs and MBPs with external displays attached.
Getting back on topic, ULTIMATELY, you'll need to decide how far you might push your craft and that will make your computer buying decision a bit easier. iMacs are relatively affordable and pretty powerful, but very limited in expansion. Mac Pros are expensive, but have a more practical usable life, especially in the professional content creation world.
