[jemfinch]

Depends on your way of hashing. If you're using linear chaining (basically, hashing a linked list of entries) you can get away with N buckets for N items if you have a good hash function. If you're using open addressing, you'll usually want your load factor (the ratio of filled slots to unfilled slots) not to be much higher than 2/3 or so. It may appear on the surface that open addressing uses more memory, but don't forget the overhead of linear chaining (an extra pointer for every element in the hash table, many more cache misses relative to a low-load open addressed table).