If your board supports DDR333, you should probably get DDR333. However, my philosophy on memory is to always get what you think will have the best longevity, so it really depends on what your long-term (9-18 month) upgrade plans are. Most of the time, the fastest memory you can afford will have the best longevity. But this could be an exception.
If you think you’ll upgrade your system to one of the first DDRII boards with dual channel support for DDR266, you might not be any better off with DDR333 or DDR4000 as you would have been with more DDR266. On the other hand, if you plan to wait for a faster version of the DDRII spec before your next upgrade, you’d be better off with faster memory.