It is important to set the maximum memory limit for 64-bit SQL Server. The memory reported as available to SQL Server includes the page file size, so if you have a page file then SQL Server will try to use it as part of its memory pool. You are then looking at what should be memory to memory transfers being disk to memory, which will badly hurt your performance.

When deciding how much memory to allocate to SQL Server, you must first take out what is needed by Windows, Anti-Virus, file cache, and other components that are always running. You can divide the rest up between your SQL instances. If they are all of the same importance, give them all the same memory. Otherwise give the most important instances the most memory.