Faster PC help needed

[conordavidthomas10]

So currently, I have an i3-3240 @3.40ghz and a gtx 660 with 6GB of ram. I also have the stock 500 GB hard drive it came with. It runs reasonably well on my 720p monitor. I was looking at upgrading, (know how to build/edit PCs so I can do it at home.) but not sure what to upgrade to see the biggest difference right away.
CPU, Monitor, Hard drive?
Just came to ask for some advice, thanks for your time!
(LOOKING FOR BEST GAMING EXPERIENCE)

 

[SgtScream]

What country are you from, do you like RGB and how much are you willing to spend?

 

[conordavidthomas10]

UK, England to be specific, whats RGB? and about £150