70 C is toasty, but OK with the 470GTX. I run 2 X GTX 470 in SLI and my idle temps sit about 30-40 C (depending on the ambient temperature). I modded out my first GTX 470 with the Zalman 3000 F aftermarket cooler and the second is a non-reference Galaxy model.
I recommend that you declock the RAM some as it is the most common cause for instability. OC'd RAM produces less of a performance boost than the speed of the GPU.
yes they run very hot i own 3 of them and on sli mode was a disaster so i decided to go with liquid cooling system which i got for free from my brothers work place.it doesnt look all that but it keeps it very cool 29c idle and 37 full load oc.