Campaign contributions for federal office: how much can an individual contribute and over what period of time?

From top of my head I think it is $2300 per individual in presidential campaigns.

Above answer is correct.

$2300 is the limit for any individual to give to any political campaign per campaign cycle.
Corporations cannot donate money to campaigns. They can, however, form PACs. PACs may donate $2300, unless they donate to more than 5 candidates, in which case they may donate $5000.

EDIT: Keep in mind: these numbers are for this (2008) campaign cycle. Under the McCain-Feingold bill, these numbers change with inflation.