You already have a preamp in your Tascam. The ART can maybe help some but it shouldn't "need" it. I can't speak to the quality of the Tascam mic, but given that it costs less than $50...
There are many, many mics out there designed to do different things, and many ways to record acoustic guitar. I like the two-mic method (a condenser and a dynamic like the Shure SM-57) or a condenser mic and line-in. In each case you're recording onto two tracks. I use an M-Audio Nova condenser and the Dean Markley ProMag Grand soundhole pickup.
Here's HT's mic page:
http://www.bulletsandbones.com/GB/Microphones.html
Also check MusiciansFriend, Sweetwater, etc.