If their 4.2J/GH number is correct, each chip is dissipating 1.3W. They aren't going to require anything special at all for cooling, likely small BGA heatsinks with pressure sensitive thermal adhesive would be fine.
Yeah, but if the chips remain stable at high clock rates while driving the voltage up, it'll need much better cooling. Also, its cheaper and saner to just use one large heatsink thats mounted on the board instead of placing the entire load on the chip itself.