I actually took it as a limitation of the technology used rather than an absolute limitation of the brain, as depicted.
Except for two key plot points:
1. In order to have enough "open memory" to store that data in the first place, he has to have HIS ENTIRE CHILDHOOD wiped from his mind.
2. When he receives a huge download of data (right to that 160 GB limit, as I recall), the entire driver for the story that follows is that, if he doesn't remove that information from his mind by a certain time, it'll spill over into the rest of his mind and FRY HIS BRAIN.