Blatantly taken from the TFTuned website:
The spring rate is the amount of force required to compress the spring one inch, and is measured in pounds. To determine which spring you need, you'll need to know which spring rate you need, and what your shock stroke is. A spring with a spring rate of 350 is easier to compress than one with a rate of 450, and is therefore softer. Springs are usually marked with their rate, and then the stroke of shock they're designed for, e.g. "350x2.75"
I guess that wraps up the 2.80 question then! Not sure about the spring rate though, 700 is pretty high - 1500 is huge! (unless mountain bike springs have a different rating system...