Putting two different speeds of ram should cause your system to use the lowest speed of the two.

If your model is a Pavilion 514x, then you should use PC2100 RAM, because that's as fast as your motherboard can use. If it's a different model, give me the exact model name and I'll look up.

For the record:
PC3200 = 400MHz on a Athlon MB, 800MHz on a P4/Celeron MB
PC2700 = idem with 333MHz / 666 MHz
PC2100 = idem with 266MHz / 533 MHz