I know this is very much like "how long is a piece of string", however...
Is it reasonable to assume that one rack server with 4GB RAM, 2 x Quad core Xeon 3GHz processors, running Windows 2003 or 2008, would be able to cope with 20TB of iSCSI storage?
This is for a new offsite backup facility, so the bottleneck is going to be the internet connection. However, if the 100Mbps leased line was working at full capacity, would the one server be able to cope?
Seems to me that it could, although I realise that it would be prudent to have a failover server of similar spec.