Issues related to RAM, Please respond to these

I have following queries related to RAM? Could anyone please explain them?
1.] What's the difference between DDR PC 2100, DDR PC 2700, DDR PC 3200 & DDR PC 4000 RAM modules?
2.] How to determine the frequency of your RAM module if it's not written in the module. I mean how to determine if it's 266 MHz or 400 MHz?
3.] What will be the impact on performance of using two DDR RAM modules of different frequency, say 400 MHz and 266 MHz?
4.] Is 533 MHz, RAM module a DDR2 RAM module?
