I dont have one what do you recommend I use? Oh and the transistors are the standard ones you get at radio shack, part number 2N3904.
A 100ohm or 1kohm is generally a good idea. It helps to prevent too much current from flowing between the I/O pin and the transistor. You have to be careful to pick a value small enough so that there is enough current to turn on the transistor but large enough to limit it. I'd start with a 1k.
Oh and I don't know if this helps or not but I'm powering the two dc motors from the 5v pin on the arduino
And how are you powering the Arduino?