vjbharadwaj,
Go to (see page 1-14):
http://dlsvr01.asus.com/pub/ASUS/mb/...6_m2npv-vm.pdf
Section 1.7.2 clearly states that you should always use memory with the same CAS latency and recommends that you use memory from the same MFG.
Clearly - if you're trying two modules of different speeds and this is not working - you need to by like pairs - same frequency, mfg, etc. If they don't make the 544 anymore, you need to buy another 667mhz module of the same make/mfg as the 667mhz module you currently own. If someone is telling you that this configuration is possible - bring the PC to them and ask them to make it work. I doubt the will be able to.
- John