Hello! Sorry for the delay.
The short answer is that it is not financially feasible. Here's why:
- The logic board is shaped a bit differently near the ports, so you would need a different frame as well as logic board (the frame is also different around the fan area, RJ-11 area, and mic/power connector area).
- The DVI port in the 1 GHz (instead of VGA in 867 MHz) means you need a new lower case to accommodate the port size difference.
- The display on the 867 MHz machine has the speakers mounted on it and the subwoofer on the logic board. The 1 GHz machine has the speakers AND subwoofer on the display, which changed logic board ports and cables on the speakers.
- There is a change from a reed switch board in the 867 MHz models to a hall effect sensor in the 1 GHz models.
- The heat sink is shaped differently in the fan area on the 1 GHz models.
- The RJ-11 board's screw hole is moved lower on the 1 GHz models.
- The fan on the 1 GHz uses a smaller connector.
- The DC-to-DC board has a different model number printed on it. It may be compatible (I haven't tested it), but it may also damage the machine.
In other words, you would need at minimum to replace the logic board, framework, subwoofer, speakers, lower case, reed switch board, heat sink, fan, and RJ-11 board. That gets expensive!
Sorry for the bad news! You're financially better off either saving for a new machine or upgrading the RAM and hard drive for better performance.