You have a couple of issues;
1)2X1GB your RAM run dual channel. 2) If you buy one 2GB and add it to the PC, then 2GB+1X1Gb, the RAM run single channel. 3) If you use only 1X 2GB, the RAM also runs single channel.
Then there is a small difference performances between CL4 and CL5.
And yes the voltage is matter too, the DDR2 RAM can even run at 2.2V. Sometime like OC if you are up the voltage ( you need test it ) then the PC will run stable.