It's all about the total current draw your external components are drawing from the arduino 5V pin. So what are actual number and value of all your pull-down resistors? How many sensors and what are their current demands? How much current does your Ethernet shield draw? When you have that all calculated or estimated then you can look at the 5V current capacity (different if USB Vs external power is used) you can tell if your hitting a limit and need a change of design, such as supplying 5vdc current with an external power supply rather then from the 5V pin. It's not rocket science but it is basic engineering.
Lefty