The T42p offers more video memory and it comes with the ATI Mobility FireGL T2 video card, while the T42 comes with the ATI Mobility Radeon 9600. Essentially the same, but the FireGL is optimized for 3D applications. As a side not, the T42p is one of the more respected laptops for gaming if that's what you're looking for.
Secondly, the two models also differ in regards to the flavors of CPU's available, although still mainly the same.