What does RAM do for a gaming computer so like a 2 or 4 GB???

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and also is Western Digital a reliable make for hard drives???

I have a Western Digital portable HDD.
It's always seemed pretty good for me, no complaints there.

There are some great answers to your question here.

But practically speaking, the reason for a slow or laggy game generally isn't RAM unless you've really cluttered up your computer. Your processor speed and graphics card determine the maximum FPS and quality your game can run at.