James,
The first thing I'd recommend is some basic exercise with your EQ and your ear. With a decent 1/2 or 1/3 octave graphic EQ (hardware or software, doesn't matter), just sit down for a half hour a night for a few nights playing a variety of music throgh the EQ while you sit there and pay attention to what happens to the sound of the various instruments in the mix as you move each slider on the EQ slowly up and down.
Use this first to learn what the various frequency bands actually *sound like*. There is no substitute for actually being able to recognize what 400Hz or 4kHz (or whatever) sounds like when you hear them (and when you don't.) Learn how to recognize key frequency bands with your ears and what they sound like on the different instruments is 95% of what one needs to use EQ well, IMHO.
Second thing I'd recommend is to do a searc on these forums for the term "parametric sweep" and read up on wherever it's metioned or described. This common EQ technique is very powerful and useful for many common EQ situations, and is also an excellent way of learning and internalizing the relationship between the numbers that you see on your EQ and the sounds that you hear in your ears.
Finally, AFTER the two things above, you might want to head over to
www.independentrecording.net, and in the "Online Resources" column, click on the link for the "Interactive Frequency Charts" for some *general* reference on some key frequency properties for each instrument.
IMHO, YMMV, ETC.
G.