There is little to gain from overclocking a 2.8GHz Celeron,
IMO it's not worth it. The motherboard you have will not overclock at all, the BIOS won't let you and it's based on a SIS 661FX chipset which will probably blow up if you try.
The only thing I can suggest to make your system faster is to upgrade the RAM to 1GB since you only have 256MB, this would be a worth while upgrade and the only one that's worth the effort and money.
With £200 you might as well build a new system, if you're up for this we can help.