With a 64-bit version of ADS, the MAX_CACHE_MEMORY setting is not useful. The value that is affected by that config setting is a 32-bit value, so it is not possible to set it to a useful value with a 64-bit server. So the best thing to do on 64-bit machines (running 64-bit ADS) is to not set the MAX_CACHE_MEMORY config value at all.
In general in the 64-bit environment, ADS will use the amount of memory that it “needs”. It will simply make allocation calls as it needs while processing queries (and performing other operations). However, it does try to limit its file cache usage some in order to “play fair” on the machine. The idea behind the file cache is to be able to pull file information directly from memory quickly, so it attempts to limit cache usage in such a way that it will generally fit in available memory. Currently, it should starts with an upper limit of 85% of total physical memory and then will adjust that downward some if other applications are using substantial portions of memory. So on a system that is only running ADS, it will use most of the memory if there is a need for it.