An SSD will give you more of a boost in performance than anything else you have listed. I could care less about lightning fast reboot times since I rarely reboot, but I would be heartbroken if I had to go back to waiting for programs to open lol. It is really that much of a difference. Is there anything currently wrong with your case, or other parts you have mentioned? Checking some prices, you could almost get a good SSD and a real good graphics card for that budget. Samsung 840 Pro EVGA GTX 660
That is a top of the line premium SSD and a really good graphics card, for about 35 more pounds......so you see what is within your range. You can get that graphics card and an SSD that is a little less expensive and fall right in your budget, AND you'll greatly increase your systems performance.
Just letting you know, not pushing anything on ya.