Under load they sit at around 70 degrees, and to overclock you simply use the AMD catalyst control centre, which has some handy sliders. You wont be able to get much more out of the Toxic 280x tho, its already running at higher clocks and cant really be pushed much more. (I think) it is faster than most other overcloacked 280x's when it is at its stock clock.
This anandtech review gives some more info http://www.anandtech.com/show/7406/the-sapphire-r9-280x-toxic-review/5
Have fun with it.