Dont let all my ramblings make you think this is at all too complicated. Its pretty simple.
A mic preamplifier simply raises the signal of the microphone(s). The way in which it does this can greatly vary from one preamp to another, based on
the design, and build quality of the circuit used in the unit.
The better the design, build quality, etc, the higher the quality of sound. Similar to an instrument.
Some can hear the differences more than others. You may not hear the difference between a 200 dollar preamp, and a 1000 dollar preamp, just as you may
not be able to hear a difference between a 500 dollar and 1500 dollar guitar. It really depends on alot of things.
However, certain things are more concrete. What we mainly want from a mic preamp (regarding classical guitar at least) is, Headroom, Low noise, and Transparency.
A quality mic preamp will give you the most out of your mics. I dont have to tell you why less noise generated by the preamp would be a good thing. You want to be able to crank your mics up without generated noise from the preamp. Cheap preamps can generate distortion even when the meters arent in the red.
Transparency = the sound is unhindered, or unchanged from microphone to recording destination. Cheap preamps add color and bumps/dips in certain frequencies that
cloud the sound. This may not be evident to you now, but if and when you upgrade to a higher quality preamp, you will hear a "blanket lifting" effect.
Headroom = headroom equals dynamic range. You want the highest ceiling you can get for dynamics. A quality preamp will handle high signal levels and low signal levels
from your mics, equally well. You may find with your current set up, that you push your signal into the red rather easily, when you turn you mics up to a healthy level, yet
when you turn them down so you dont overload the recorder, the signal then seems too weak, and doesnt pick up quiet nuances as well. This is an example of limited dynamic range.
Just understand that, i am talking from the audiophile perspective here. Its totally possible that you could buy the RNP mic preamp mentioned here, and not hear any difference from your current set up. It really depends on how discerning your ears are, and how picky you are about noise, dynamic range, etc etc...
Judging by your current gear choices, and the fact that you're a classical guitarist, i would have to assume, you would probably hear quite a big difference.
The Tascam recorder is very nice. And the NT5's are also very nice mics. I think a nice preamp would serve you really well.
If 475 is above your budget, i would suggest maybe one of the stereo ART preamps. They arent "fabulous", but they would certainly give you more "beef" than your current set up. Checkout the M-audio DMP 3. A very nice 2 channel mic pre for under 200.00 usd.
Though i would also add, you may want to watch my video tutorial on this site entitled "Recording Tutorial" viewtopic.php?f=16&t=44972&hilit=recording+tutorial
which shows you how the room affects your recording, and some
things you can do to remedy the pitfalls of small, rectangular, drywalled rooms. When you step up your preamp, you will most certainly begin to hear more of the room
you're recording in, and this can be good or bad depending on your room.
Best of luck!!