What Capacities Do Databases Have?

razor69 asks: "Having programmed PHP in conjunction with MySQL for some time now, I'm quite happy with the whole setup. However, I'm now working up to some more serious clients, which require larger and larger databases. Looking for database comparisons, I'm stuck with the stats MySQL give me and none of the database manufacturers tell me what the maximum amount of records their system can handle is. Now I know that Oracle is unlimited, but so is their price. Which database program is suitable for what price, how many records can they hold (practically as well as theoretically) and how do they all REALLY compare?"