The first program I linked above-Newertech Gauge Pro-actually looks to do a decent job.
I installed it on my GigE, and it initially reported a temperature of 100ºF, which seemed pretty reasonable given that I had used the computer a little bit and just installed a program.
I fired up Photoshop to convert the OS 9 screen shots to .png. As you can see, at least initially Photoshop didn't get it that warm. I opened a photo, however, and applied a few filters to it. I managed to get it up to 110ºF, which I think is pretty reasonable. As far as I know Photoshop 7 is actually smart enough to use both processors, and it also lacks some of the really resource-draining stuff of later versions like distortion correction.
I think that CS 2(which does have a distortion correction filter) will run in 10.2.8, so if I had a temperature monitor app that would work in 10.2 I could try to really push the CPU and see where the temperatures went.
I've been using complex integrals in Mathematica to benchmark 604Es, so it might be a good candidate for pushing temperatures up also. The version I'm using is old enough that I'm sure it won't use Altivec, and I also don't know if it multiprocessor aware(installing it on my 9600/200MP should be able to answer the multiprocessor question pretty easily, as I have some times for it on a single 200mhz 8600).