Any faster grade DDR memory will work. Names are:
PC2100 same as DDR 266 mhz, usually CL 2.0 on wait states
PC2700 same as DDR 333 mhz, usually CL 2.5 on wait states
PC3200 same as DDR 400 mhz, usulally CL 3.0 on wait states
Run sandra on your RAM and see what your wait state is (CL 2.0, CL 2.5, etc.).
If you have CL 2.0 make sure the new stuff is 266, CL 2.0 or 333, CL 2.5 then you can run both sticks at 266 mhz CL 2.0. You will not get any advantage to buying just one stick of faster (333, 400, etc) RAM so unless you plan on upgrading the MB soon don't bother.
If your MB supports faster RAM the alternate approach is to buy a stick of 512 mb 333 mhz RAM and sell the old RAM. The price difference between 266 and 333 is not much anymore and the price of 512 sticks is dropping. You won't see much if any speed difference between 266 mhz and 333 mhz. The big advantage comes in laod times with the extra 256 MBs and not having to cache memory to disk.
Note - The faster the memory the bigger the wait states. 266 @ CL 2.0 is almost as fast as 333 at CL2.5. If you want higher speed or smaller wait states you pay for it whether you can use it or not.