your internet experience with the Quadra 650 will depend on the amount of RAM installed rather than processor speed. With 16 MB of RAM (i.e. physical RAM) or more, it will be not too bad. Do not rely on Virtual RAM or RAM Doubler, if you want good performance.
In addition to what wlonh says, the modem you use will be important, too. Try to get a fast, good one (e.g. Global Village Teleport 33.6 or 56k).
Hope this helps,