Since I don't know your rig , I cant say exact parts to buy , etc.
But I can answer you -
RAM in Gaming - RAM helps in saving all stuff of the game like game engine and data which needs frequent access. These days 8GB of RAM is quite good for games. I say buy 12GB ( 3x4GB ) RAM of 1600MHz or 1866MHz with Latency below 10. Few tips for buying RAM - Check these points before buying a RAM -
1. Frequency and CAS Latency - Frequency is the speed of RAM. The faster it is , the quicker the operation the operation. CAS Latency is the delay time between the moment a memory controller tells the memory module to access a particular memory column on a RAM module, and the moment the data from the given array location is available on the module's output pins. In general, the lower the CAS latency, the better. These two factors affect the RAM performance. For Home users these things don't matter. For Games , The Frequency for a great gaming experience is 1600MHz with Latency below 10. For Domination , 2133MHz on Latency below 10 is perfect.
2. Capacity and Modules - For standard use no more than 4GB with a single or two RAM sticks. For gaming minimum 4GB is recommended. You can decide how many modules by choosing how much GB of RAM you want. For e.g - If you want 8GB , 2 Modules of 4GB is good enough. Never fill up all the slots when your building a Rig. For optimal performance get a single module no more than 4GB and have as much as sticks as possible.
3. Buy a RAM with Heat sink for Gaming.
Recommended RAM manufacturers - Corsair , G.Skill , Crucial , Kingston.
SSD - SSD's are way too faster than HDD's. This is because of several reasons. Lets move to there uses in gaming. Due to their lightning fast speeds , SSD's are great for storing games. They will load games quick compared to traditional HDD's. They have low power consumption compared to HDD's which helps in increasing battery life for Laptop users. SSD's will start games quick and load quick and since they have no spinning parts , they are durable and will keep your data safe compared to HDD's.
Hope I helped.