The total maximum budget for the whole pc would be usefull to know......along with what bits you already have.
Are you looking to build a complete system or just a new base unit?
What about a graphics card? Good quality here is vital in a gaming pc......do you already have a card or is the budget you stated above meant to include graphics?
For example you could get an SLI equipped motherboard by a decent manufacturer such a Gigabyte and an Athlon 64 3500+ cpu for £250 if it just for cpu/motherboard.
Plenty of prices to caompare at click here