By default the proprietary driver for Nvidia cards on Linux fires an interrupt for every frame drawn on screen, whether it needs to or not. This increases power consumption. To avoid this use version 100.14.19 or later of the driver (the nvidia-glx-new package in Ubuntu 8.04 provides 169.12) and set OnDemandVBlankInterrupts to true in /etc/X11/xorg.conf. For example:

Section "Device"
Identifier      "Configured Video Device"
Driver          "nvidia"
Option          "NoLogo"        "True"
Option          "OnDemandVBlankInterrupts"      "True"
EndSection

This will reduce the time your CPU spends on spurious interrupts. Your laptop should run cooler and longer as a result.