Okay dumb question (since I'm not a complete noob)
I have a servo on its own power supply (raw port of the arduino)
I need to be able to turn on and off the power to the servo using the arduino.
I'm thinking about putting a resistor to the servo power line, then have something with a variable resistance (like a potentiometers) that control with the arduino (thus making resistance higher or lower than the resistor to the servo)
For this kind of ON/OFF switches, MOSFET might be even better as they are voltage controlled and you wouldn't need to worry about the current capacities of the output pins.
relay transistor fet, can all switch a load on or off, they all have unique operating stats and choosing what YOU need really depends on what your trying to do with it
ie a 2222 transistor would be overkill to switch a led, but would be "insta-smoked" trying to switch a average sized light bulb
Would something like that blow a transistor? Would I be safer with a relay switch?
Depending on the load on the servo, it might draw more than 800 mA. You should measure this first. Do this by having the servo turn, while trying to stall the output shaft (depending on the size of the servo, this might not be possible, or the servo might be damaged - use care) while you measure the current by putting the meter in between the positive voltage pin of the servo and the servo's power source.
Once you have done that, you can then size the transistor, FET, -or- relay (relays can have problems too if you don't size them right) to the load of the servo. Let us know what you measure for the current, then we possibly can help you further.
My servo is drawing 5v and maybe up to 800mA (the max the wall wart is rated).
Would something like that blow a transistor?
That depends on the specific transistor you select, they come in all voltage and current ratings. It should be no problem finding a transistor rated at say a couple of amps max current flow. Now to control flow from that 800 amp wall wart might take a while looking through catalogs and datasheets.