technically, if you have a mixer with preamps, you don't NEED to get a seperate preamp unit (unless you have a multi input interface and need more mic inputs). Depending on what kind of mixer it is, most of the low end ART stuff wont yield any better results anyway.
People like to use external units because they can be of much better quality and they want different types of preamps for different types of sounds or applications (clean, tube, etc.). Some are better suited for vocals, some for acoustic guitars, etc....
I would suggest if you don't have much of a budget to pick up a seperate mic pre, use the mixer's pres for now and save up and get a decent mic preamp.....
