cabecinhas:
It won't generate 6V, however it will get more out of your batteries and provide more output current. It can take 5.5V max on its input.Seems to be what I'm looking for... But.. question.. where to find it? do u have an idea?
Trust me, unless you have very tight constraint on weight the cheapest and more effective solution is to use 4 AA or AAA cells. Voltage boosters aren't cost effective in your case because if you hope in saving on batteries cost by boosting the voltage of two consider you'll discharge them a little more than twice the speed.
If you need 6V to power your arduino consider powering it with 4.5V (3xAA or 3xAAA) through the Vin pin.
Any conversion/boosting will waste part of your battery energy for the conversion.