It should be noted that while Ilford publishes this and that the truth is that there is no such thing as "pushing" film. That is to say, that if you underexpose you can NOT overdevelop to get the detail you did not expose for.
The zone system is based on this fact, or set of facts. If you don't expose enough, you can develop the film for a year and you will get nothing more in the shadow areas. These are fully developed at about three minutes and all you will add will be base fog.
That said, what does get added is at the top end, or the darker parts of the negative. In zone system terms, one controls these with the amt of development time so that you don't "blow out" the highlights. The more you develop the darker the highlights are (lighter in the print). Therefore, what you are doing when you take your film to the lab and ask the guys to "push" it is you are adding a little contrast at the top end.
For most images, losing a stop of detail is not critical and your 400 ISO won't be a big problem.
Finally, I know there are a number of people that like Rodinal. However, in my opinion, it is not a developer for people starting out. It has a very specific effect, which is to coat the edges of grains with a darker edge so that it can look like its sharper. However, if you enlarge it past a certain amount, you will see grain the size of golfballs. It also is awful for scanning. If you like to see a sea of grain in your image (that's an aesthetic choice and everyone gets to make it themselves) then by all means go ahead and use it. However, be aware it is not a general purpose developer and will limit what you can do with the film.
Lenny
EigerStudios