I'd recommend reading the BOL article on this DMV first:

http://msdn.microsoft.com/en-us/library/ms175019(v=sql.105).aspx

single_pages_kb is one specific type of memory allocation that's actually part of the buffer pool anyway, so you won't see any use of it for the buffer pool itself.

Virtual_memory_committed_kb is probably the most useful column to order by if you're looking for what's using all the memory...

Virtual memory's not always directly related to physical memory, but assuming pages are locked in memory then yes.

Bear in mind that SQL Server won't de-allocate memory it's already taken if it doesn't need to, so it's normal that it will rise to it's maximum value after a period of time and doesn't necessarily mean it's under memory pressure.