Hahahahaha @ Hzza

This
If you're looking to spend wisely, its got to be on the new family of Intel processors (i5 range looking best bet for your budget).
You probably need to look at an i5 processor, P55 motherboard (think that's right) and some new DDR3 ram. Should come inside (or close) to your £300 budget.
Note that existing stuff should transfer over (4850 and use existing case), but you'd need to re-install the operating system. If you don't have a re-install disc or the software is tied to your current PC (as with OEM versions of Windows), you may need to fork out for Windows again (Windows 7 probably).