Cores =/= performance, lower end Intel dual cores outperform plenty of similarly priced AMD quad cores. There's no clear cut way to say your processor will be 2x better than your old one. It's just not that simple. I had an Athlon II X4 620 @ 3.5GHz with 8GB of DDR3 1333 RAM, and my processor maxed out with only like 6.5GB of RAM used. I also multitasked so that's including Chrome, MSN, Windows itself, etc. It also varies from bot to bot how much RAM it will be using but I'm willing to bet that unless you do some serious overclocking on that proc that your proc will max out before your RAM. Not trying to say I'm the end all be all of computer shit, but speaking from previous similar experiences, I'm pretty certain of it.
If you get it stuck in your head that you're gonna need more RAM though, here's a quick/easy explanation:
Your motherboard will either have 2, 4, or 6 RAM slots. Most have 4. Your prebuilt will probably have 2x4GB sticks, leaving an empty 2 slots. All you need to do is find out what your RAM speed/specs are (usually listed somewhere on the module itself) and try your best to match it. For example if you have DDR3 1333 already installed, try to buy more DDR3 1333. Also, don't bother buying single sticks if you have two slots available. Having dual channel RAM is worth the little bit extra you'll pay, because it drastically increases your efficiency. The actual process of putting it in the computer is simple. Just take off your side panel, find the RAM slots, open the little hinges, and put the module in the slot. It may take some pressure, but not a ton. They are made so they only go in one way, and it should be pretty easy for you to visually see which way it has to go without much explanation. Make sure it is fully seated (the hinges should click back in place as you push it in) before turning your system back on. That's basically it.