Core duo is the latest offering and the fastest
Celeron is quite an old name but it is usually a cut down version of another processor these include Pentium 4s and other CPUs.
Pentium M (often called centrino but only when used with certain wireless gear) is the middle of the road processor. Before Core duo it was the flagship mobile processor.
If all you are after is internet access the currys 399 offerings will do you more than well enough. And from what people have been saying tesco is doing a 299 machine but it is hard to come by. However I cannot tell you more as I live in ROI till saturday.
If you can elaborate a bit more on what you plan to use the machine for we could make more suggestions however basically for internet any laptop will do.
Do you play games, encode video, like to watch movies etc etc and we can make more suggestions. Is it for business or pleasure. Will it be used in a hard environment?