Dual-Channel ISDN(whatever that is...) is 128kbps.(~12kb/sec max) My Mom's office computer downloads 5.4kb/sec(INSANE 56k!). Both are slow though, so I'd define broadband as anything at or over 256kbps.
Pre-Broadband: 0 - 128kbps
Slow Broadband: 256 - 768kbps
Normal Broadband: 1.5 - 4mbps
Fast Broadband: 5 - 9mbps
In Canada though the slowest broadband ADSL connections are 1.5mbps(with options up to 4mbps). Cable connections vary greatly, from 512kbps to 6mbps.