Can I add faster memory to my motherboard?

My motherboard can take memory up to PC2100 DDR. I have installed a 512Mb
chip. I would like to add more memory. But I don't want to waste money if I
can't use this memory in my next motherboard.

So:

1. Can I install faster memory (PC2700?) into my mobo?
2. If not, can I use my slower (PC2100) memory in my next mobo?
3. If so, will this slower memory (in my next mobo) greatly harm
performance?

If I can't install faster memory in my current system and can't transfer my
memory to my new mobo then I'll just wait to I upgrade before buying more
memory.

Cheers.

Bobby