The system will not Boot or will not Post?
If it will not post I have seen this previously where the RAM isn't suitable for the M'Board. In those cases the M'Board doesn't like using 4 sticks of RAM but will happily use 2 X 2 GIG Sticks and not 4 X 1 GIG Sticks. If you go to higher speed RAM on all the RAM that you use with the Intel P43-P45 Chipsets at least you can then populate all 4 RAM Sockets but those M'Boards do not work with more than 2 X Slower RAM fitted. Not sure if the same applies to NVidia Chip Sets but the symptoms are very similar.
In the past when I've needed to install more than 2 RAM Sticks I have gone to the Highest Speed Possible and then fitted 4 of those without a problem. Unfortunately ASUS doesn't list any Compatible RAM on their Web Site so there is no guide to use when looking at what you can fit.
Download the Qualified Vendors List {Memory} from here
http://www.asus.com/product.aspx?P_ID=mBDSvKdwaNonTlOn&content=specifications
And follow their reconditions as to which RAM to use in the configuration that you want to use. But previously the Owners have opted to go with 2 X 2 GIG Sticks as this has worked out cheaper for them.
Col