There are basically two types: PCI and USB. The former are better for gamers while the latter have mobility and are more popular.
All modems are designed for the maximum speeds per service, and for the 512K service each will achieve this in ideal conditions. However, some modems are better performers than others in poor conditions and generally its the drivers that have the greater, or lesser, effect.
E.g. Alcatel drivers have good reputation while Globespan don't seem to be their equal for PPPoA (ATM) protocol.
The first thing you must do is decide on an ISP and find out from them what modems they support.
Just a few thoughts.
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