You need a motherboard that will let you, by way of divider settings. Some motherboards will give you options like 1-1, or 3-4.
From those settings....let's say your FSB is 133. At the 1-1 setting, your memory is also at 133. At the 3-4 setting, your FSB is still 133, but your memory runs at about 170. That would be the only way. Unless you're well over 133mhz FSB, that PC-2100 should be fine. Chances are you aren't taking full advantage of that PC-2700.