I have been struggling with the same problem (same motherboard same RAM upgrade even down to the make Crucial) and while reading through this forum I also thought the voltage was a good idea.
In my motherboard manual it refers to a DDR Voltage setting switch JP10, it is located at the inside end of the AGP slot (I had to remove my graphics card to get access to it). This switch is set at a default of 2.5v (pins 1+2)but can be used to switch to 2.6 (pins 2+3).
After switching my system seems to have accepted the two 512 sticks anyway this is the longest period its EVER ran with 1GB of RAM. Goodluck.

There are many things to consider when buying memory.
Crucial memory is only "guaranteed" to work if its on the list from the "Memory Advisor™ tool" based on the motherboard used.
http://www.crucial.com/store/listmfgr.asp?cat=RAM
"We guarantee the upgrades you find through our Crucial Memory Advisor will be a perfect match for your computer."

Compare the memory you have from the list of compatible memory from Crucial for the ASUS A7A266-E.
http://www.crucial.com/store/listpa...rds&mfr=ASUS&cat=RAM&model=A7A266-E&submit=Go

The voltage the memory bus is running at should be listed in the motherboard BIOS.
But I am skeptical of the accuracy of the sensors of many motherboards.

"The 184-pin DIMM must be a 2.5V unbuffered DDR SDRAM."
http://www.asus.com/pub/asus/mb/socka/m1647/a7a266-e/a7a266e-100.pdf

http://usa.asus.com/download/mbdriver/soketa.htm

Thanks for that Triton.

You must have guessed that changing the voltage switch wouldn't be a permanent solution. Back to a freezing system.

The most my motherboard seems willing to accept is 768mb - being an original 256 & one of the new 512's I hoped to upgrade to.
I checked out each link you provided & confirmed the crucial memory cards I have are on their recommended list 184 PIN DIMM DDR PC2100.
Re-set the voltage switch to 2.5 - unless someone has any better ideas I am giving up & making do with my 768mb.