Mastering in garage band

I have a song that I'm ready to master. Several questions - 1) should I master in GB using one of the presets and call it a day? 2) should I go outside of GB and run the song through my Mackie 1604VLZ board, 3630 compressor and BBE sonic maximizer and then reimport to GB OR 3) does it make sense to master it in GB then send it to iTunes and then bring it back into GB and tweak it again with yet another master track in GB? There are a lot of real and software instrument tracks, but my machine seems to be handling it ok.

I'm running GB '08 on a 2008 17" MBPro with Snow Leapord Ver 10.6.3. I have 4G of RAM. Thank you so much for your help!