1.67 to 1.70v is moderately safe...1.68v max for 24/7?
1.73-ish for benching

And here are some links to kits for those in the US who can not afford Corsairs $250+ prices:
http://www.ewiz.com/query.php?dp=1&d...=%23displaytop
http://www.ewiz.com/query.php?dp=1&d...=%23displaytop
http://www.ewiz.com/detail.php?name=...8x8-CL6-Memory