I have a sixteen year old home located outside of Atlanta. It is once again time to paint. However, I notice some of the people in the neighborhood have re-sided with HardiPlank and some have painted existing siding. I am told the determining factor when it comes to re-siding vs painting is the swelling of the planks. I currently have Georgia Pacific siding and would like to know what constitutes swelling (i.e. width in inches) so I can make an informed decision.
The answer really depends on what kind of siding you have. If your siding is wood or composite siding - which it sounds like it might be - it's going to need to be repainted on a regular basis, regardless of how obvious the swelling is. HardiePlank, on the other hand, is not organic - it's concrete siding. It doesn't suffer the same wear and tear as wood siding because it doesn't soak water, swell, or rot. It's very stable, and paint lasts longer on it.
My house has cedar siding on it, but my garage - built more recently - has HardiePlank in a cedar pattern. From the street, the two are indistinguishable. I was able to order the HardiPlank pre-painted, so it was already primed and painted when it arrived. Because it's factory painted, it lasts a heck of a lot longer. If you have to replace your siding, this is a great choice - getting it prepainted from the factory. It's a bigger upfront investment than repainting your siding, but it'll save you big in time and money if you plan to stay in your house awhile.
If your existing siding is swelling at all, I recommend replacing it. It's not getting better.