I wonder why it stresses the graphics card.
It might be because of the type of visualizer it is. Personally, I've noticed a difference between how "shiny" the iTunes visualizer is compared to all others (even G-Force) and iTunes takes the cake for aesthetics alone. Sure, it might not have the most algorithms or anything, but they look good. I'm wondering if that causes the integrated GPU to start doing work, which casues the CPU to heat up due to the fact that 13" MBP's have integrated graphics.