This is really a question of how databases use memory.
When a query engine is executing a statement, there are times when memory is used to store transient structures like spools, table variables, and so on. Under a lot of memory pressure, this stuff often ends up being written to tempdb (in the case of SQL server), which adds i/o overhead.
For a specific case, you really need to look at your own usage profiles to see if extra memory would help.