Modems of 56 kbps take advantage of the fact that many telephone networks already have been partly upgraded to digital telephone switches, and that the connection between the phone company and your Internet service provider (ISP) usually flows through a digital line.
These digital lines are cleaner and allow data transmission at higher speeds with fewer errors than regular analogue lines.
A regular modem will simply transmit data in analogue wave form over the whole distance, only converting the information back to digital 1s and 0s at either end. A 56 kbps modem also boosts its signal to fight line noise.
Downstream from the ISP to the user, the 56 kbps modem probably won't exceed 40 kbps, mostly due to Hong Kong's telecoms infrastructure not being up to the standard of the West. Upstream from the user to the ISP, the 56 kbps modem is limited by its technology from exceeding 33.6 kbps.
If you plan to upgrade your 28.8 kbps modem, you might as well skip the 33.6 kbps modem and go for the 56K. Prices are similar, with some as low as $1,300.
So far, there are two 56 kbps standards, with US Robotics on one side with its x2, and practically the rest of the industry on the other with the K56Flex.