Get a digispeed (Australian) - look it up on the web - very good service by post - worked first time - you only need the cheaper one - the DC01.
This connects to the PWM output of the computer - and converts the signal directly to a voltage. The voltage depends on what voltage you want - so if you put 10 volts in, the output will be between 0 and 10 volts - depending on the pwm.
The two parts of the board are electrically isolated, so there is no danger of anything going wrong.
Only uses one output pin.