We also need a budget. Besides the CPU keep in mind you'll have to upgrade motherboard and RAM as well and possibly the power supply depending on what you want to build.
Hi sparkdex,
we need your motherboard make and model to know what CPU's it will take. I take it your in the UK, as you said you have 100-200 quid to spend?
also what PSU are you using?
and if its a gaming machine, a GPU/Ram upgrade may be a better choice.
Your specs should include the hard drive, memory type, and age of both, as well as the power supply.
It appears you simply do not have enough ram, but otherwise, may be ok if the power supply is up to snuff.