i woudn't try that if i were you... because DDR2100 is running at 266Mhz and DDR2700 is running at 300Mhz. You'd better buy 256Mo DDR2100 or 512Mo DDR2700.

But maybe some1 will be able to tell you if DDR2700 can still work at 266Mhz...

also check that your Mobo has a 300Mhz FSB, otherwise you won't be able to use DDR2700 on it.

I have a 256MB stick of DDR2100 - if I buy a 256MB stick of DDR2700 (my mobo is happy running the ram at 333mhz) am I likely to run into any conflicts because my RAM is clocked at different speeds?

Just to get it rigth: Your mobo is capable of running 333 MHz Memory (PC2700) but you use it at 266 Mhz for your PC2100 DIMM. If that is right you can insert a PC2700 DIMM additionally without problems, but you won't be able to run it at 333 MHz, it will operate at the same speed as the old one. Cause if you would run your memory bus at 333 MHz the PC2100 Dimm would very likely fail to work at all. And it is technically impossible to run one DIMM at 333 MHz and another one at 266 MHz.
So you could buy yet another PC2100 DIMM or replace your old memory completely by 512 MB PC2700. Then you could use 333 MHz memory clock.

As I.C. said, you can do it but you'll only be able to run both at 266 (multiple ram is always run at the speed of the slowest stick) otherwise you'll be overclocking that 266 ram to 333, and im not sure many can manage that.

Also note that some mainboards arn't happy with two unmatched sticks, it's rare, but you might pick up a few problems.

sorry for my mistake, PC2700 is running at 333Mhz not 300Mhz.

Argh anyone want to buy a 256MB stick of DDR2100 then? :P