PC2-6400 is the same speed as DDR2-800, which uses a 400MHz clock frequency. Different MBs use different menus to set up memory speed, so you may also want to check with Asus support.
The other thing to remember is that your RAM is only guaranteed to run at 4-4-4-12 timings under a specific voltage, usually higher than the standard 1.8V. Since you are running it at 1.8V now, you will need to run memtest86+ for many hours to make sure the RAM is stable once you get it running at DDR2-800, or you will need to increase the memory voltage to the level specified by g.skill. Either the RAM packaging, the g.skill website, or their tech support can tell you the memory voltage needed.