If you are interfacing the Wixel with a part running at the same voltage, you don't need shifters. However, I was under the impression the minimum recommended operating voltage for the Arduino Uno is 7 V. Furthermore, according to the datasheet, an ATmega328 running at 16 MHz requires at least 4.5 V to be in spec (you can run it at lower voltages if you keep the clock at 8 MHz or less). Separately, I think you'd be better off boosting a single AA battery to 3.3 V with a regulator or using two cells with a sepic or buck-boost regulator than powering your circuits directly from two AA cells.
- Ben