Because you aren't on the bleeding edge, reliability is excellent. Documents are distributed randomly across the nodes. Eric says his network is nowhere near capacity. All the nodes at his site run the same software and speak HTTP to handle user queries. Databases are stored on RAID disks and paged into shared memory.
Processors handle queries independently on a shared database.
To scale to more users, these sites simply replicate the database. Replication also provides fault tolerance, which is effective, but expensive. Inktomi also has partner sites in Australia and Japan. Each partner site also runs a crawler on its geographic part of the Web. Data are transferred between sites on magnetic tape because shipping magtape gives more bandwidth more cheaply.