Ram is temporary memory. When you use data or an application stored from your hard drives, your computer will load the data from your hard drives onto your ram. Ram memory is a lot faster and gives better performance but is limited in capacity. An analogy I've seen is like taking food out of the fridge and placing it on the counter to prep before cooking. In this example, the fridge would be the hard drive and the counter is the ram. It's a lot easier to work with the food on the counter than in your fridge.
So there are two main things to consider when it comes to ram, capacity and speed. Your first question is one of capacity. Keeping with the kitchen example, having more is like having more counter space. You can load up more data to work with at a given time.
Ram also has a speed specification. The speed (frequency) is usually indicated in Mhz (e.g. 1333 Mhz, 1600 Mhz, 1866 Mhz, etc.). This indicates the maximum bandwidth which is how much data is can be transferred at a given time. Again keeping with the kitchen analogy, its how much stuff you can carry from your fridge. So the higher the better. There's also latency which is indicated in timings (e.g. 9-9-9-24). Generally speaking, the lower the better but don't worry too much about this.
8 Gb is more than enough for almost all the games currently available. PCpartpicker has a compatibility filter so all the rams it display after you have selected a motherboard would work. Go for a dual channel kit (2X4 Gb) as your motherboard can support this. It will offer better performance. For 50 to 60 USD you can probably find a 1333 Mhz or 1600 Mhz pair like
http://uk.pcpartpicker.com/part/corsair-memory-cmv8gx3m2a1600c11