Depends on which chipset the motherboard uses.
Memory speeds explained:
PC1600=200MHz DDR
PC2100=266MHz DDR
PC2700=333MHz DDR
PC3000=350MHz DDR
PC3200=400MHz DDR
PC3500=433MHz DDR
The Celeron/P4 line works out as follows:
4x (system bus)=FSB
4x 100=400
4x 133=533
4x 200=800
Although you should be able to get away with PC1600, if you upgrade to a newer P4 with a 533MHz or 800MHz FSB, you'll either cap your bandwith or be unable to use the PC1600. Anymore, PC2100 is the slowest you'll find, but PC2700 isn't that much more expensive.
The faster RAM will work fine at slower speeds, so you could go buy PC3500, and have no problems. It would be paying much more than necessary for extra RAM.
I'd take into account your current motherboard's capabilities, if you plan to upgrade to a newer P4, and if that P4 upgrade would require the purchase of a motherboard to go with it.
This is a general guide. We can help narrow down what's best if you can give a little more info about your system, such as the motherboard (or chipset----check device manager). Also, how much RAM you have now and what OS you're running.