DAC

I've had a whole list of projects that I'd like to use my brand-spanking-new Arduino board for and now that it's arrived, it's time to get started. Hopefully, the first thing I'd like to do isn't too ambitious... I'd like to use the Arduino, basically, as a DAC: to convert numerical data from my PC into voltage, via serial. After briefly thumbing through the reference, I was wondering how to go about it? I had planned on running a shell script that would query my computer for the amount of ram used, strip everything out and spit a number (percentage used) out of the serial port. This, in turn, would be read by the Arduino and it would feed voltage to a LED ladder that would be a bar-graph, lighting up (let's say 10) LEDs to indicate the percentage of RAM utilized.

--- EDIT ---

After a bit more reading I figured out a way to do it: I'll just send voltage out on multiple pins to correspond with what is input from the serial. So now my actual question is, what should be sent out of the serial port from my computer to the Arduino? Binary, hex? Thanks in advance!