Hmmm, personally i'd say avoid PC world like the plague. Any local computer shop will generally beat their price and anywhere decent online will beat that.
With the limited budget it's hard to say what will make the biggest difference but it would be nice to know the graphics card too.
What you COULD do is:
<can't post links as a new poster>
All from ebuyer and total is £131ish
The bonus here is you get a really heavilly feature restricted i3 (which smokes the core 2 duo upgrades being suggested) while buying board and ram that are i5/i7 sandybridge compatible. You simply need to drop a big processor and a better graphics card on this when you get your money and you are golden.
The board a BIT basic but as you say your PC knowledge is limited playing with overclocking etc isn't needed (which is what better boards offer). The ram is also cheap n cheerful (but from a relatively decent brand at least) but again, unless you know how to tweak and push your hardware is fine for your needs.
Least I checked that chip smokes most of the core2duo's as well (it's basically a really feature trimmed i3). A pretty randomly googled review of the chips: <same again>
If you wanted the bottom end i3 chip is £20 more and JUST meets the top end of your budget
There's not much in it though