Typically faster RAM will scale down to slower speeds just fine. There are the odd exceptions to this, but generally it'll work just fine.
If you installed PC2100 RAM, you would slow down the RAM you already have installed. So while your system would still be quite a bit faster on account of slow RAM being better than not enough RAM, unless there's a significant price difference it's not worth intentionally hindering your system's performance.
Of course you didn't say what OS you were running. Your saying "physical memory" makes me think maybe you're still running one of the old DOS based Windows versions like 98 or Me. If that's the case, adding more RAM will NOT help your problem. It takes a little time getting your head around the idea, but that "physical memory" in 95/98/Me refers to the first 1MB of RAM. It's an old DOS legacy that has been haunting people for decades, and finally ended with WindowsXP that brought the NT lineage to the masses.