Intel release some problematic drivers. Since they are "open-source" friendly, they made their way into X, and some kernels.
I also have the 845 chipset. It was particularly hard hot by this. The 865 fared marginally better, while the 915-955 got by quite acceptably, but were lacking proper OpenGL support.
The kernel in the 2009.4 release has updated drivers, and it fares quite a bit better (doesn't hurt that LXDE is more resource friendly than KDE, either
, so I never did migrate back to KDE on that chipset).