On the power supply side, ATX 2.03 includes a new kind of power supply specification called ATX12V. ATX12V power supplies will have two main differences from normal ATX power supplies:

There is a new +12V power connector and more +12 VDC output capability. This connector will provide more usable power to the CPU area of the motherboard. The Pentium 4 is a high-power processor and, by having a second power connector, motherboard manufacturers won't have to route a large amount of power across several crowded inches of motherboard. Power will instead be delivered closer to where it is needed. ATX12V cases will also need to provide more +5 VSB current. The minimum is now 1.0 amps with 2.0 amps preferred.

ATX12V cases will be fully compatible with systems that need previous ATX power supplies. In other words, an ATX12V case would work fine with most of today's motherboards. However, an ATX power supply will not work with a motherboard that needs the extra +12 V power connector.

You can read the full ATX12V and ATX 2.03 specifications here.