The CPU CAN NOT be upgraded. At least not in the G4 series.
RAM can be. They use single sticks; G4's use standard desktop style DDR1, intel's use laptop style (I'm pretty sure they're DDR2)
Hard drive can be upgraded with any standard laptop IDE drive. Newer Intel based ones may be SATA, but still 2.5" for sure. I've heard of people using larger drives, but it requires case modifications and you must modify the port (at least with IDE)
Can't upgrade the video card. It's onboard and there's no expansion slots inside.
__________________
Urban De-Development
* Athlon X2 4200 | 2048M RAM | 160G HD | 7600GT
* C2D T5250 | 2048M RAM | 160G HD | Intel x3100
* G4 1.33GHz | 512M RAM | 40G HD | Radeon 9200
|