No 4200 & 4300 are excatly the same both 533MHz. As to answer your second question google is your friend, but read this here:http://www.cooltechzone.com/index.ph...d=377&Itemid=0
or for just the part about cas here you go, although reading the whole article will help:
CAS is Column Address Strobe or Column Address Select. CAS controls the amount of time (in cycles (2, 2.5,& 3) between receiving a command and acting on that command. Since CAS primarily controls the location of HEX addresses, or memory columns, within the memory matrix, this is the most important timing to set as low as your system will stably accept it. There are both rows and columns inside a memory matrix. When the request is first electronically set on the memory pins, the first triggered response is tRAS (Active to Precharge Delay). Data requested electronically is precharge, and the memory actually going to initiate RAS is activation. Once tRAS is active, RAS, or Row Address Strobe begins to find one half of the address for the required data. Once the row is located, tRCD is initiated, cycles out, and then the exact HEX location of the data required is accessed via CAS. The time between CAS start and CAS end is the CAS latency. Since CAS is the last stage in actually finding the proper data, it's the most important step of memory timing.