Can I run six 595 shift registers (SN74HC595NE4) with the 5volt pin on the diecimila? Or would it be better to power them through a 12v to 5v regulator?
Each pin on the shift registers will be used to turn on and off transistors.
How many mA do the 595's draw? Could just use a resistor divider network to get about 6v from the 12v?
Should be no problem at all just using the Arduino +5volt supply. If you look at a data sheet for those chips the actual current used by the chip is well under 1 ma each. Where the current usage can increase is the things you will be driving with the outputs of those chips. 6 chips would give you 48 outputs so you can see the current use can increase quickly depending on what you are driving with all those outputs. If you drive logic level FET switching transistors then no worry. If you are directly driving 48 LEDs at 20ma each then you use up all the 1 amp 5v power that the Arduino 5v regulator can handle.
So those 6 chips driving transistors should be no problem with using board power, but it's always best to review the final circuit to tell for sure.
Lefty
Could just use a resistor divider network to get about 6v from the 12v?
No, the supply impedance you get would be too high and you will burn up too much power and the voltage will change as the load changes. Use a regulator IC it so simple nowadays.