FPS stands for Feet Per Second, for those of you who hate anything other than SI units (Systeme Internationale), one fps is 0.304 meters/sec, (equivalently 1m/s = 3.28 fps). This is why 328fps is thrown around as a limit. 328fps is 100m/s, and a 0.2g bb travelling at this speed possess the energy of exactly 1 Joule.
Energy (in joules) = 1/2 mass * velovity^2
How can I calculate my airsoft replica's energy?
Using this handy form you can calculate your replica's energy in joules. As a byproduct it'll also give you your bb's velocity in m/s. To calculate the energy in joules, simply enter the mass of ammunition (in grams) that you use, and the fps that you've read from your chrono unit.
So how does extra FPS affect my range? Well I've been having a think about this, and we will be conducting some tests with some AEG's at some point in the warmer part of the summer, from these we'll be able to deduce some realworld figures. Right now though the easiest thing that I can do is calculate some range using basic physics, making certain assumptions. For starters we'll ignore hopup, as we know it increases range, and I'm not going to sit here and work out higher level physics using specific air density and fluid dynamics  I'll do that later on =). This model assumes a curved flight path, I know hopup produces a straighter path, but you'll be suprised how close the figures from this model match to reallife.
From basic physics, lets start with a basic equation:
s
= distance u = initial velocity v = final velocity a = acceleration t = time passed 
s = ut + 1/2 at^2

Assume you are firing the bb from a height of 1metre (i.e. with your rifle shouldered),
lets calculate the time it takes for your bb to drop and hit the ground:
s = ut + 1/2
at^2 hence, 
Hence time taken to fall 1 metre is 0.45 seconds. With a velocity of 328fps or 100m/s, this mean your bb will travel (328*0.45)=148feet or 45metres before hitting the ground, giving you your effective range. If you don't know the energy of your rifle, you can either calculate it above, or use 1J as the limit for AEG's, and 2.31J as the limit for single action.