I'm so frustrated on getting 24GB of ram detected that I'm thinking about abandoning this build. I was using the Asus P6T-Deluxe with my X5660 and 24GB of ram. It worked fine with 24GB for about 1 yr and it suddenly it was only able to detect 16GB. I put up with the missing 8GB for a year and the again, the amount of ram detected dropped to 8GB. I tried reseating the ram, CPU, checked for bent pins, wiped the contacts of the CPU with alcohol, tried a number of different ram settings. No bueno. I ended up buying another mobo, Asrock X58 Extreme. The exact same issue happened.
I tried with only 3 sticks of ram plugged in and it only detects 8GB. The slots that work and don't work changes depending on the number of sticks of ram I have plugged in, so the issue isn't associated with a specific slot. This suggests the issue isn't with the CPU pins.