Different power sources for pcb
It is normal practice (as in the Arduino, for instance) to have multiple stages of voltage regulation.

For instance, on the Arduino, the incoming voltage (from the barrel jack) is first regulated down to 5V, with a limit (purely due to the components chosen) of around 800mA.  That 5V is then further regulated down using another voltage regulator to 3.3V, with (again because of the components chosen) a current limit of 300mA.

So from that single power input you get both your 5V and 3.3V regulated outputs.  The current available in the 5V rail is dependant on the current being drawn by the 3.3V rail, so if you have 50mA being drawn by the 3.3V rail the 5V rail would have 750mA available to it1.

So when designing your board you need to have a clear idea of what current requirements you will actually have, and what voltages are needed.  De-rating (increasing the rated value above what is needed) with voltage regulators is often a good idea as it can reduce the amount of heat generated (and thus the size of any heat sinks).



1 Note: high current consumption causes high temperatures.  Although the components are rated at 800mA it doesn't follow that you can use all 800mA as it may melt!