There are good mics in a number of price catagories. For acousitc guitar, several electret condenser mics have a nice, open sound.
As important as the mic, itself, is the quality of the mic preamp, which is a specialized low noise circuit that must take a lot of gain (up to 60 - 70 dB) while retaining bandwidth. For example, the output impedance of a typical mic is around 150 - 200 ohms. The theoretical noise of an ideal 150 ohm resistor is -131.5 dBm at room temperature. I have built mic preamps with in equivalent input noise of around 6 dB (input shorted), which means that most mics are noiser than my preamp. This design is also flat to around 400 kHz.
Can you hear the difference? You can if the rest of your recording chain is good enough.

In professional recording environments, I have heard the differences between various mics, through the same preamp, and the same mic through various preamps, all of which had at least 100 - 200 kHz bandwidth.
Could you hear the difference when recording to MP-3, or even CD (uncompressed 44 kHz x 16 bits)? Probably not. :disgust: