DDR ram is backwards compatible -- you can mix PC2700 and PC2100 just fine, they will both end up running at PC2100 speeds. Depending on the motherboard, you may need the slowest stick in your first RAM slot.
One catch: make sure you're not mixing DDR and DDR2 / DDR3, etc... The DDR2 / DDR3 run at a lower voltage than the standard DDR ram, and plugging a DDR2 chip in a DDR slot would fry them. (If they even fit, not sure about that) but since this is PC2700, it's more than likely regular DDR memory, so should be fine.
One catch: make sure you're not mixing DDR and DDR2 / DDR3, etc... The DDR2 / DDR3 run at a lower voltage than the standard DDR ram, and plugging a DDR2 chip in a DDR slot would fry them. (If they even fit, not sure about that) but since this is PC2700, it's more than likely regular DDR memory, so should be fine.