Yea, it's huge. My first HDTV was a plasma. It was like somebody finally put color back into life. Games are often colorful and dark, especially nowadays, and so there's really no medium where these image quality aspects are more important.
I felt similarly ecstatic when I changed my TN monitor to an IPS, though it lacked black levels I wanted.
I really dont think you need to upgrading anything else with the display you are using. When people are saying the CPU is a bottleneck for you, they aren't saying you have a poor CPU and should upgrade it, they are merely saying that at 720p, your GPU is so underutilized that CPU becomes the primary bottleneck to more performance. But do you need more performance if you're already at 60fps in most any game? Does your TV even go up to 100hz or more? If it doesn't, maybe I can understand, but otherwise, if you're at native resolution and 60fps, I'm not sure why you'd need anything better than what you already had.
Your CPU is not bad by any means. A 4670k will be able to push 33% higher clocks or so, but it's not going to be a gamechanger. I wouldn't say it's really worth the extra £200 plus installation hassle when Haswell is not even latest CPU tech, but again - up to you to decide. I feel like maybe you have very different ideas of 'value' when it comes to getting more performance.