The 3GB limit is the maximum amount of memory that can be addressed under 32-bit operating systems.  It doesn't matter where the blocks are.  32-bit operating systems are also limited to a max of 4GB total, however it is split up.  This is the big advantage of switching to a 64-bit operating system - the limit is very much larger, so large that it's pretty unlikely you will see hardware that approaches that limit for some years to come.
http://msdn.microsoft.com has much information more accurate than I can convey it.  Basically all that stuff about what hardware grabs which memory is no longer a concern

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