Does the ram /always/ show up as 16 gig in your bios?
Also 16 gig and "triple channel" don't really sound right togeather. (Or did you mean 18gig of triple channel?) If 16, gig probably should be dual channel, though that I believe is a function entirely of the MB itself, except that dual/triple channel ram is generaly sold in matched modules packs in sets of 2 or 3 and are best installed in those groupings.
Are you sure the ram you got was vetted by ASUS? (ASUS lists compatible modules in their manuals).
Edit, also are your computer specs up to date? Still using the core 2 quad?