The diff between 400MHz and 333MHz RAM is exactly as the term indicates. The former is 20% faster than the latter. Overall system performance will be slightly better with 400MHz memory but not by 20%.
The difference in price is very small, so it's well worth going for the faster memory.
One point of clarification : these RAMs do not actually run at 400 and 333MHz. The basic clock frequencies are half of these figures, i.e., 200MHz and 166MHz respectively. However, DDR (Double Data Rate)RAMs process data twice in each clock cycle, so that they do their stuff at twice the basic clock speed. The proper term is DDR400 and DDR333. They are also called PC3200 and PC2700 RAMs.
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