I am helping my friend choose a RAM as he wants to upgrade it 1Gb. He already has a 512mb installed and the RAM his motherboard can support is up to DDR 400/PC3200. What he wants is to add another 512mb of RAM. I was abit concern so can you guys help me out with a few doubts that i have:-

1.) How would i know if it is compatible with the pre-installed RAM. What are the factors involve?
2.) I saw that scan has a 30 pound 512 CL2.5 RAM for sale, if the pre-installed RAM is a CL3.0 what will be the CL speed of the two RAM when installed?