The last bios for the Asus A8N-E was released on May 3, 2006, bios version 1013. It's possible that his board is simply starting to fail due to age. The board was released 6 years ago, with a maximum memory support of 4GB.
A8N is an older board. Make sure you have a 64bit bios on it. You may also need to change the memory remap setting in the bios.
Does the bios see all 4GBS?
Bios is only 8-bit....
Heny is right, check to make sure both sticks work. Test them one at a time. If they both do, you need to update your bios. Hopefully on exists that you can use 4GBs of ram with that board. If not, you're SOL.
The board supports a maximum of 4GB with the initial bios firmware. If he's using 2, 2GB sticks, he's lucky it even posts. The A8N-E only supports 1GB per slot, which means 4x1GB sticks to reach 4GB (page 1-15 of the Motherboard's User Manual).