I just started using the Arduino Uno and got the hang of it. However, my project requires much more pins, and I am going to purchase the Arduino Mega. If this is the case, would I have to purchase new USB cables, power source hook ups, etc.?
The standard Mega uses the same cable and power source as a standard Uno and the gear is interchangeable.
I don't know what the overall power limit is but the power per pin is the same 40Ma, and I imagine the overall is too. Since you want the pins, an external power source for what is on them may be necessary.
If you are using the USB cable for power, it might be a good idea to get a wall wart.