The 2 minimum bypasses are 100nF, .1 uF capacitors one from Vcc to ground and one from AVcc to ground close to the chip which at least on the 28 pin devices are across from each other so a 100nF/.1uF cap across pins 7 and 8 (Vcc) and 20 and 22 (AVcc). Adding a 10 to 22 uF cap couldn't hurt... Unless you put it in backwards... Messy and stinky, that.
Bob