5.1 Types of Compressors:
Compression and limiting is a way to control the dynamics of a track or mix. It can be used to lower signal peaks close to the threshold you set. You determine how close the level conforms to the threshold by setting the compression ratio. This is the amount the signal is compressed to keep it close to threshold. Most of the time, strive for the "Less is more" category. Compression can help tighten and define your sound, but you shouldn't "hear" the compression. It should be as transparent as possible. That said, there are quite a few good compressors in the $500 range. The compressors in this price range are generally very usable, unlike some of the less expensive ones. These are more transparent, and can offer better results with higher compression ratios. Although there are still much nicer models on the market, they demand a much higher price.
Look for used dbx 160's. They're a half rack wide and 2 spaces high. These are a very nice compressor (mono) and can be had for $600 or so used. A pair mount side-by-side in a rack, and they may be connected to a stero pair with a jumper between them.
The DBX 1066 is a stereo compressor/limiter. It's very quiet, flexible enough to use on most tracks, and works surprisingly well.
The Aphex 651 Expressor is a mono compressor/expander. The expander focuses on restoring the high end from a heavily compressed track. This allows a little more squeezing than you could do with some compressors. (Although I don't necessarily recommend heavy compression in most instances)
Also look at the Aphex Compellor. This is available used for this price range. They're designed primarily for compressing mixes, and are very clean.
The Drawmer DL 241 is another good compressor that can be found used for around $500. It's a step above many of these others.
A hot new compressor is the RNC from FMR Audio. This compressor is around $200 and blows away most everything here. Very transparent sounding. Comes as a stereo unit in a 1/3 rack mount case. There's only one pair of controls, so it can not be used as a dual mono unit.
5.2 Attack and Release settings:
Setting the attack speed of your compression helps determine exactly what
the effect will sound like. Using a faster time will allow you to compress
peaks at the beginning of words and phrases. This can be hard consonant sounds
like T's and popped P's. Doing this may muddy up the sound a little, by rounding
out the inital sound, and making it unintelligible. In other words, use the
longest attack time possible that will allow you to achieve the proper results.
For release times, you have to think of the nature of the sound again. If
you need to quiet down a short burst or excalamation but not affect the following
phrases of lower volume, then use a shorter release time. This will allow
the compression to release quickly with out smashing the quieter portion.
Do some phrases start at lower level and swell? For these, use a slower attack
and a longer release.
For the appropriate amount of compression begin between 2:1 and 6:1. There
are times you may want to squash it more than that, but not often. It will
become unnatural sounding if you compress much higher than that. Finally,
you won't go wrong at first just putting all the knobs straight up and driving
it like that until your own ears start to tell you what they would rather
hear. In experimentation, extreme settings can demonstrate characteristics
in a way that makes them easier to hear even if those settings aren't what
you might wish to use regularly.