If just talking of 64-bit architecture then it is 16 exabytes which equals to 16,000,000,000 gigabyte. 16 billion GB
32-bit and 64-bit are just ,how can i say in a simple way, processor design and architecture and how they process data. 64-bit is helpful in video trans-coding, encryption etc.
And Apple are not stuck on 1GB RAM , they choose so. 2GB RAM phones are also in market nowadays.