Q: What and 4 gb Ram module should I buy to upgrade my iMac model id.10.1 built in late 2009?

I know all the specs from my computer but there is such a wide variety of memory modules on the market to purchase, I am confused as to which parameters are critical.

For example latency, do I need a 7 or a 9.

My original MHz is 1067, is 1066 OK?

Is any MB size fine as long as it is between 256 and 512?

Is 1.35 volts or 5 volts the correct voltage or does it matter?

I have been offered to buy a PC3-8500 versus my original PC3-10600, is there a discernible difference? 

Are there some reputable brands that you would recommend?  I can't seem to find real Apple brands.

I have been trying Amazon but that is why I am now here asking.

Thanks!