Expansion chambers do exactly what burnzy says, they are designed so that any liquid CO2 from the bottle is turned into gas before it gets to the gun. It makes the gun more consistant and makes the seals last longer..
Do you need one?
Well it depends on the type of bottle you have, some CO2 bottles come with an anti-syphon tube inside them that stops the bottle feeding liquid CO2. In the old days they rattled if they had a syphon tube in.
If a gun has a bottom line then it really shouldn't need an expansion chamber as the tubing will do the job.
On a 98 personally I wouldn't bother, you are better off saving the money you'd spend on upgrades and putting it towards a higher spec gun.