64-bit allows access to greater amounts of memory available to programs. 32-bit is (for the most part) locked to the 2gb limit per program.
The 2gb thing is a Microsoft licensing problem...
Go to professional 32 bit windows OS and you don't need a 64 bit OS... go to http://en.wikipedia.org/wiki/Physical_Address_Extension#Microsoft_Windows and look at the 32 bits limit... Well windows 2000 32 bit datacenter was already able to access 32 gb ram...
In fact, since the pentium pro from 1995, processor are 36 bits... and the today called 64 bits processor are in fact 48 bits... Original x86-64 was 40 bits, can be extended to 52 bits in future processor generation using the same architecture...
By the way, original windows xp was able to use more that 4 gb ram by using PAE... but these feature was removed with the service pack 2 and limit the OS to only 4 gb ram...
Well, windows server 2003 32 bits have a limit of 128GB and Win7 x64 home have a limit of 8 gb ( premium at 16 gb and from the pro version, 192 gb )...
So, sure, for people having more that 128gb ram, it is a good thing to move on a 64 bits OS...
For the curious, read the article at http://www.geoffchappell.com/notes/windows/license/memory.htm ...
Sure, there is reason to upgrade your OS or system... by example, computer processor have evolve from MMX to AVX ( http://software.intel.com/en-us/avx/ ) via SSE, SSE2, SSE3, SSSE3, SSE4, XOP, FMA4, CVT16... old OS don't support new features from hardware ( by example, i need to create a boot USB with driver for my SAS system else the install CD of XP pro x64 cannot find any of my 8 SATA drive connected via the SAS interface )... directx10 and up don't exist for XP... unless a hack version is used... look at the picture of FSX at http://www.techmixer.com/download-directx-10-for-windows-xp/ ... see the difference between directx 9 and directx10 ( until now, there is no way ( hack ) to use directx11 with XP ...
So, there is plenty of reason for upgrade to newer OS or hardware but not the usual reason like the memory limit... who is not a hardware limit but a Microsoft licensing limit... In fact, people using Linux 32 bit or MAC OS X 32 bit don't have these 2 or 4gb limit !!!