EDIT: The PC-266 is an error on the part of whomever wrote the manual. DDR-266 is actually PC2100
There is no PC266:
SDRAM DIMMs - These first synchronous registered DRAM DIMMs had the same bus frequency for data, address and control lines.
* PC66 = 66 MHz
* PC100 = 100 MHz
* PC133 = 133 MHz
DDR SDRAM (DDR1) SDRAM DIMMs - DIMMs based on Double Data Rate (DDR) DRAM have data but not the strobe at double the rate of the clock. This is achieved by clocking on both the rising and falling edge of the data strobes.
* PC1600 = 200 MHz data & strobe / 100 MHz clock for address and control
* PC2100 = 266 MHz data & strobe / 133 MHz clock for address and control
* PC2700 = 333 MHz data & strobe / 166 MHz clock for address and control
* PC3200 = 400 MHz data & strobe / 200 MHz clock for address and control
read this:
http://en.wikipedia.org/wiki/DIMM
if it is really DDR-266 like i said, then anything between DDR-200 (PC-1600) and DDR-400 (PC-3200) will work on it. If you put in something UNDER 266 is will slow down the entire computer (your CPU and FSB will be slowed down to match your ram). IF it is over 266 then the ram itself will slow down to match the rest of the system. and just run at 266.
Make absolutely sure the new ram you buy matches the current ram and the specs in terms of "registered" and "buffered" as well as ECC status.
http://en.wikipedia.org/wiki/Registered_memory
http://en.wikipedia.org/wiki/E...rror-correcting_memory
http://en.wikipedia.org/wiki/Fully_Buffered_DIMM
For example:
http://www.newegg.com/Product/...x?Item=N82E16820148120
This ram is registered & ECC ram 4GB (a pair of 2GB sticks) that is DDR1-400. If your system requires ECC & registered DDR1-266 then it will work on your system at 266 speed.
Being slowed down is will not harm it, in fact it will make this ram will take less electricity, produce less heat, and last longer before failing (probably forever considering the situation)