hi jonathan
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The 5v cominoug from arduino when connected to the USB are supplied from the computer. this means that if you overload it you risk damaging the computer (altough it's a very remote risk)
Whenever you plan to power anything bigger than a couple of LED use an external power supply. Anyway motors (unless tiny) should not be powered from the regulated 5V supply but should use the unregulated voltage supplied to the board (this usually floats between 7.5 and 9v) -
Going from memory the OSC pin determines the PWM frequency used to drive the motors. if you change those values you change the pwm freq.
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a trimpot is a potentiometre that can be adjusted with a screwdriver. connect one end of the trimpot to 5v, the other end to ground and the pin in the middle to vref. tweak until the motor runs smoothly
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You can use it with unipolars. you'll have to hook up the motor in a different way. (look http://www.st.com/stonline/products/literature/an/1734.pdf here for details)
you can drive the 298 directly from arduino if you dont need any of the sofisticated features it has.
for simple circuits a nice L293D (D not E or B) has power drivers and diodes integrated.. you can see a circuit here http://www.potemkin.org/cms/Pid/StepperMotor
massimo