Hello,
250 GBP means roughly 375 USD.
You can go for:
- i5-4460 (180$);
- H97 motherboard (85$);
- Corsair Vengeance 8GB (2x4) DDR3-1600 CL9 (75$).
Only prebuilt systems came with the components already mounted (RAM included). If you don't want to risk anything, address a local computershop to assemble the RAM and the CPU onto the motherboard.