GO ASUS, asus is 10 times better than anybody except ABIT 3 years warranty and stuff so i don't think somebody who knows ASUS and gigabyte would say gigabyte
I've used many ASUS and Gigabyte motherboards in the past, and they both offer excellent quality. It's all a matter of choice, though where I live, the 8KNXP is a tad more expensive than the P4P800.
I have been usign the same gigabyt board for about 3 years now and its solid has an intregrared Ide promise card that allows for 8 IDE devices in sted of the standard 4. I like gigabyet just seems they have a few nice extras others don't but Asus is also top notch you should be safe going with either. Just picka board that has all the features you need.