PC2100 refers to the bandwidth of the memory chip, in this case it can shift 2.1GB per second (or 2100MB-'ish', I know I'll get flamed for that statement!). PC2700 in turn has a bandwidth of 2.7GB per second.
If you install some PC2700 memory, your computer will use it as PC2100 memory if that's as fast as your motherboard will allow. In your case, it's more important to make sure it's the SODIMM type that'll fit your laptop and not the regular PC stuff that won't.
hanks for that answer, as long as I get 200pin DDR SODIMM memory I should be ok, would the size still be the same i.e. If I get the 512mb PC2100 - It should fit?