Keep in mind that there are 8 bits to each Byte. In my case, the cable modem I used to have advertised upload bandwidth of 384 kbps (kilobits per second).
384 kbits / 8 bits per Byte = 48 kBytes (KB)
The most I ever could send upstream was about 39 KB/sec, which is about right considering network overhead, imperfect conditions, etc. This was with a DI-704 I bought off ebay for $10. I think it's the same as the DI-604, just in metal box instead of plastic.
When you see "kbps" and "kb" you have to try to figure out of they're talking about bytes or bits. Most of the time a Byte is represented by the capital "B" and a bit is represented by a lowercase "b", but not always.