Cisco stores them as 32 bits, but SNMP also retrieves them as 32 bits. HP
has a proprietary MIB & impl. for its Lanprobes & RMON s/w for HP
workstations, which is 64 bits wide. These are the only devices I know of
that store more than 32 bits, and have accurate counts of bytes over a long
period of time.
So on ciscos, you should clear counters before they can roll over.
You should really have a raffle to guess the uptime on your router. 
    --Dean