70C isn't bad but if it gets above 80C then you're having a real cooling issue. You can try maxing our your GPU and seeing what the temp is, download FurMark and do some testing: http://www.ozone3d.net/benchmarks/fur/
Also, make sure your computer in general is free of dust. Open it up and see if the fans are covered in dust, check the back of your case too. If it is, clean it out with a duster and compressed air.