Apple doesn't want you to upgrade your computer - they want you to buy a new computer!
There is some expandability with some of their systems, but the "Apple-branded" hardware usually costs a lot more than comparable PC hardware.
Over the years, Apple has made Macs more and more (and more) like a PC. An Apple computer now pretty much *is* an i386-compatible PC, capable of booting Windows right up.
Over the years, as Apple computers became more PC-like, they gained the ability to use many pieces of hardware that were designed for PCs or even Windows.
Mac compatible hardware (no modification needed):
Bluetooth 2.0 EDR adapter, USB, 300feet/100meter range, cirago BTA-6060
Bluetooth 2.0 EDR micro adapter, USB, 100feet/30meter range, cirago BTA-3210
802.11b/g 54 Mbps WiFi adapter, PCI (seen to the system as an AirPort Extreme card), ASUS WL-138g V2
USB 2.0 card, PCI, IOGEAR GIC251U
20X Dual Layer capable DVD-Burner (IDE), Samsung SH-S202J
Hardware compatible with software modifications (flashing):
Multiple NVidia GeForce and ATI Radeon video cards, such as:
- GeForce 2 MX 400
- GeForce 2 MX 400 TwinView
- GeForce 6200
- Radeon 9700 Pro
See my Mac Video Card page for more information.
Hardware compatible with physical modifications:
Thermaltake TR2 W0070RUC 430W ATX Power Supply
It's size, shape, and wattage make it a popular choice for replacement G4 power supplies. It is used by many people in their G4 Sawtooth, Gigabit, Digital Audio, Quicksilver and Mirror Drive Door Power Macs.
It requires modification, as Apple decided to alter the ATX layout for different Macs.
Info on ATX supplies and the variations/changes Apple has made: