First, you have to decide, what kind of PC you want to build.
1. Office and Media PC
If you want to build an Office PC and are REALLY low on Budget, Gb of Ram might be enough, but this is only enough for really basic Text writing. If you w ant to play Movies and watch 1080p Internet Videos, you should go with 4 Gb of Memory, this will make sure, that it won't lag and the prizes for RAM are actually really low, so it shouldn't really matter, if you choose 4 and not just 2 Gb.
2. Gaming PC
If you want to build a gaming PC, with which you can play new games at high Settings, for the next year and at normal settings for a couple of years, you should definitely go with 8 Gb of Ram.
If you want to you can also go with 16 Gb, this will be helpfull, when rendering videos and editing images. But it's not necessary for normal gaming.
3. High-End Gaming PC
If you want to build a High-End gaming PC, with which you can play games at highest settings for the next few years, you should go with 16Gb, 32 Gb might be too much, but if you want to, it won't b wrong.
If you want to build a Workstation and want to edit videos and pictures in 4K, you should go with 32 Gb of memory. 64 Gb is probably too much. You should rather invest the money in a better CPU or GPU...
After deciding, how much RAM you will get, you have to decide, which RAM to get.
You should definitely get DDR-3 RAM, no matter, what kind of PC you'll get.
If you get a normal Office and Media PC, the basic 1066 Mhz RAM.
When you are planning on a Gaming PC, you can go with the 1066 Mhz Ram, but that might be a bit too slow, so you should, if possible, go with the 1333, or 1600 Mhz RAM.
If you want to build a High-End gaming rig, you really should go with the 1866 Mhz one. If you have enough Money, you can even go with the 2000 Mhz mmemory. But the RAM Speed is pretty unimportant. You should get a better CPU, then a higher Speed Rate. But if you want to and have a bit of money left, always go with the faster RAM.
I hope, I could help some of you peple out there.
P.S: Sry for my not really perfect English, but I'm german