This CPU cooler is the best within your budget,OcUK Imp MicroATX Tower Case
Dimensions (mm): 180mm x 360mm x 425mm (W x H x D)
So it does have a little head room.
Budget is about £20 max but if there is something thats loads quieter a little more I can stretch the budget.
Sorry misread, thought you were replacing the fan that's on the heatsinkThats a 92mm fan i need an 80mm plus thats louder.