If you know the FSB of your computer then you can figure out the speed of it because it is equal to the fsb, but it is probably ddr ram so you divide that by 2. IE my fsb is 800 mhz so each stick is ddr 400. you can also go into the hardware profile in the windows control panel and it will tell you. Pins really is irrelevant, I think its 148 pins but you dont need to know really if you just plan on buying a new stick. Yes you can mix different brands of ram, be sure to get the right speed though.
As a question to add on to this for anyone who wants to answer, what happens if you put in RAM that has a greater clock speed or lower clock speed of you FSB? Will a slower stick cause all other sticks to be accessed slower?