I would go for a sempron over a celeron.
How about click here click here
thats £354 in total
The motherboard is a socket 754, older, but it will take a lot of other cpu's if you ever wish to upgrade.
The extra 256Mb of ram will help alot. 512Mb extra would be even better.
The motherboard does have a 8X graphics slot, so it would take a reasonable graphics card, if medium gaming was ever needed.