JCL is correct, its best to make your particle animations as simple as possible unless you have an 'super computer' and/or the accurate simulation of rain is an important element to your game (and not just an effect). To fix the problem of having rain fall only to certain heights you can set up "rain zones". Each zone covers a certain part of the map and has a unique Z range (i.e. zone over ground stops at Z = 0, the zone over a building stops at Z=500). I did something like this in the upcoming tech-demo.

btw: I know it looks pleasing to have some randomness, but don't raindrops all fall at the same rate? Maybe not, I've been in SoCal too long so I've forgotten. Anybody have rain where they are?