An overview of the ASUS P5K PRO is shown in the two images below. While it may look like your typical motherboard from a distance, taking a closer look reveals a bunch of small differences; some of which may not be so welcome. To cover the basics, you have an LGA775 CPU socket, four DDR2 memory slots, two PCI Express x16 slots (the blue one is x16 electrically, while the black one is x4 max electrically), two PCI Express x1 slots, and two PCI slots. Large Copper heatsinks covering the northbridge, southbridge, and MOSFET transistors around the CPU socket stand out against the dark PCB.
The next image takes a look at the back panel connections. From left to right there are PS2 keyboard and mouse connections, two USB ports, a S/PDIF out connection, a Firewire port, two more USB ports, a 10/100 Ethernet port, another pair of USB ports, and finally a bank of six stereo jacks for the integrated HD audio. On the plus side, I am glad to see PS2 connections included, as some boards have eliminated them all together, or simply provide one. From an organizational point of view I would prefer all the audio connections to be grouped together, and consolidating the USB ports would be nice, too. eSATA has recently made its way to the rear panel of many boards, but not on the P5K PRO, so I will have to find another way to connect my exteral hard drive.
One significant difference that I actually didn't notice until I tried to put this board in a case is that it is about 2.5cm narrower than a typical ATX unit. Your typical board will measure about 24.5cm wide, while this one is about 22.0 cm. I'm not sure if the reduced size is part of the 'green' push, but it could be since just over 10% of the PCB has been eliminated. One thing the reduced size does impact is the placement of various features.
The below left image details a bunch of items that are located in less than ideal positions, most likely due to the decrease in overall board real estate. The main issue is with the placement of the 20+4 pin power connection, which has been moved clear across the board from where you would expect to find it. You will need to get creative with cable management, as it has to reach down below the CPU cooler, which might not look so nice through a case side window. In the same general area is where you will also find the CPU fan power connector, front panel HD audio header, and CD audio input header. I would prefer the CPU fan header to be near the top edge of the board, and the audio headers to be near the bottom edge of the board. I haven't connected anything yet, but I am already picturing a mess of wires to get everything connected.
The CPU socket is closely surrounded by components of the 8-phase power design and somewhat large Copper heatsinks. The northbridge chipset cooler seems to have an especially large foot print, and is located very close to the CPU socket. Toward the edge of the board are four 240-pin DDR2 slots which let the P5K PRO accept up to 8GB of memory.