I have a pretty nice rig. Intel Core 2 Duo 3GHz, 8GB RAM, 5HDDs so that I can capture video to a separate HDD and also output the final video to a separate HDD (reduce HDD load), and a nice 3D graphics card (ATI Radeon HD 3870 512MB).
With all of that, my most recent 17-minute HD video took over 2 hours and 40 minutes to output. I output first as a QuickTime movie, and then use that movie with Compressor to make my final output version for YouTube/Vimeo HD.
I recently did a 25 minute video and it took upwards of 6+ hours (I didn't output first as a QuickTime movie).
Although I'm editing in FCP, I'm not really adding much more than what you would add in iMovie (transitions, titles, that's about it for the most part). You will find it INCREDIBLY slow and painful. Trust me.
It's also not wise to capture video to the same drive running FCP and Mac OS X. You should make sure to capture to a second drive, so install an eSATA card and grab yourself an external drive.
In case you're unaware, you
*CANNOT* use USB to work with HD video. It just cannot handle the data flow, PERIOD. A USB external hard drive is essentially worthless for video editing (unless you just manually backup videos at your own leisure).
Also, be wary of daisy-chaining an HDD to your Firewire with your camera at the same time. Both devices running simultaneous while capturing footage may cause issues. I don't recommend it.
BTW, I have an HV20 and it's awesome
P.S. Remember, you can set the camera to DV mode and just record at SD quality. You would use the same MiniDV tapes for HDV/DV (don't bother with the special HDV tapes, waste of money).