As for RAM, 8GB is definitely more than enough. 1600 MHz is also good enough (running myself), otherwise both of these are quality companies and RAM manufacturers often also include lifetime warranty, so it's your call, which one you like.
Reading from internet forums, I have noticed rather large amount of negative responses regarding Seagate and their durability. I have been running a 250GB one and 2TB one for 2 years now, no issues so far. I would recommend going for something more quality, such as Western Digital, Hitachi, as this is quite a vital part of your PC.
For motherboard, just pick one that fits in your budget in the end and has the features you like/need. There is no performance difference.