kinda depends on your current setup, because depending on what you get you might need a new motherboard and ram.
if your socket A then i'd get an AMD Barton 3200+ from just under £100 >
http://www.overclockers.co.uk/acatal...acp_2d058_2dam
if you have more money than that then go for 64 bit >
http://www.overclockers.co.uk/acatalog/AMD_64_Bit.html
basically the best thing to do is find out the max amount u want to spend and get something at that price. otherwise you will be tempted to spend less and get something that will not last as long.
personally i would go with AMD, their prices are cheaper.
but as i said before i dont know your current setup so you might have an intel board