Why not keep track of temps in the game itself? Afterburner has an on screen display that can be shown in the corner of the game. You can monitor temps, framerate, fan speed, GPU usage percentage (that can track how well SLI is used), etc.
One 460 is enough to play on high, but two should be able to run on ultra. Although you will probably run into a vram issue with only 768 MB (SLI does not double the vram). So you might have to turn down some things, but most would be on ultra. This is assuming 1080p. For lesser you could do all ultra with AA, and for higher you would probably have to go high and even medium in some cases (that is when the vram limit really kicks in).
If you ever wonder where you stand vs. today's cards, Anandtech has a comparison tool. They don't have 2 768 460's in SLI, but they do have 2 1GB 460's (you will perform very similar). The performance is just like a GTX580, even a little better.
http://www.anandtech.com/bench/Product/314?vs=305