I've recently purchased an awesome i5+P7P55D+arctic 7 freezer pro bundle from one of my friends, but oddly enough, he wasn't able to sell me his RAM.... However, I managed to pick up a Kingston 2GB DDR3 HyperX 1800mhz 2GB stick for an unbeatable and unresistable price, but upon doing some research and a bit of reading on the manufacturer's website, the Asus P7P55D doesn't seem to state memory support of 1800mhz RAM (only 1600 or 2200?).
First, without some BIOS work, the i5 only "supports" 1333MHz ram. Also, it can only use 1.65V ram (that ram says 1.7 to 1.9V). However, this is fine because you can easily run the ram under spec. It will default to some low (1066 or 1333) speed and 1.5V for compatibility. You could leave it there, or try a setting like 1333, 8 8 8 24, and 1.5-1.65V. You may have to do a lot of guess and check (and BIOS resets) but you should be able to get it working. As you can see, I am running my 1600 speed ram at 1333 and at tighter timings and lower voltages, so it is quite doable.