PC2100 runs at 133mhz (DDR Effective 266mhz) and it has a bandwith of 2.1gb/s (hence 2100)
PC2700 runs at 166mhz (DDR Effective 333mhz) and it has a bandwith of 2.7gb/s (hence 2700)
In order to run PC2700 at 166mhz you need a board that supports it such as the Via KT333 or SiS 745. If you put it in an older board it will just run at 133mhz or whatever speed the board supports (so long as it supports DDR RAM)
I have a KT333 board but only have PC2100 in it, but at 18 Euro's you may as well get the PC2700, even if your current board doesn't support it, you can run it at PC2100 speeds and then use it to it's full potential later.
Corm