Firstly I probably wouldn't buy from PC World as their components are over priced I would buy on-line from click here you will get more for your money.
Another 256 of ram would be useful although any more might be a waste unless you are doing memory hungry tasks such as video editing.
I would go for a 128 graphics card I just bought a radeon 92000 for £40.
If you are going to get a more powerful processor make sure your motherboard will take it.
You could go to your local computer store and get advice from someone who knows what they are talking about so that you get the proper components.
P.S. What m/b do you have on your system?