An AD20 will provide 17dB of minimum gain as well as the analog to digital conversion. Pre-amps in general are used for gain, bringing a mic-level signal to line level, as well as phantom power.
I suggest to just keep it simple, get a pre-amp that provides phantom power, less boxes, and less hassle. The MP-2 mentioned above would be a sufficient deal, however they sell for around $400 when they show up in the Yard Sale here, and they are gone real quick!
Seriosly, I wish I'd know about this site before I bought all my gear, I would have done things a lot differently. It took me forever to figure this all out on my own, so a +T for finding it.