So, pretty much after seeing this, i got cancer from the replies.
People are saying that 1600MHz is much much better than 1333MHz with RAM, and although that is true, it's completely not worth paying more as there isnt a massively notable difference.
Also mixing a stick with 1600MHz and one with 1333MHz isn't a good idea at all, it is more efficient if they both operate at the same speed, so my suggestion is just to get 2x4GB sticks of 1333MHz RAM as you already have.
And in regards to your HDD,
Yes, this is the best thing to do, 1TB would be the best. SDD isn't needed as it costs quite a bit and it doesn't look like you want to spend the money on an SSD.
The Graphics card that you have at the moment isn't the best either, maybe an R9 270x? it's relatively cheap and works a dream (i have one).
Nvidia is also a very good choice but it costs more money.
If you have a big budget then yes get Nvidia, but if you don't then get an AMD graphics card.
I would also recommend using this website if you are building a custom PC, it has loads of advice on components based around budget, it will also help you understand all the things that you probably don't understand at the moment: http://www.logicalincrements.com/
http://linustechtips.com/main/ is also a good forum to ask questions about nearly everything about computers, there are like 1000+ people on at a time and you will get replied to in 5mins