With careful tweaking its possible to undervolt and overclock your cards at the same time, if you know what you're doing. For example...
2x290P in crossfire, typical stock clocks for 290's are 947/1250, mine come clocked at 975/1250. Through a lot of tweaking and testing i found im able to lower the core voltage to -0.031v in afterburer whilst having the core of both cards overclocked to 1050/1250. This is actually cooler and faster running than stock 975/1250 and stock voltage. I recommend you all experiement and try for yourself, can make a big difference.
You can also lower AUV voltage as well, this will help temps even further. If you start lowering AUX though its best to leave the memory clock at stock. I have my AUX running at -0.100mv and i have pretty good temps now on my reference cards with a much lower fan profile that typically needed to keep temps in the low 80's.