I've done that myself with my home built system, except I use EVGA Precision X. Most of the tweaks I did to the default curve were on the low end of the scale - making the fan run faster at lower temperatures. It makes the computer a little noisier when doing normal stuff, but blowing off that excess heat does make the idle temp come down, which seems to move the entire temperature profile down.
But I have to agree with Jasper, especially since the machine is still under warranty. There's no reason for the GPU to idle at nearly 70 oC, and that will significantly shorten the lifespan of the card and other components. If you can't get those temps down one way or another, you need to contact Dell.