A 32-bit OS is limited to 4GB of memory.  The reason you won't see more than (approximately) 3.2GB is because the OS uses the top memory addresses (working downward) for addressing video memory and hardware on the system.  The more video memory and different peices of hardware in the computer, the more of the upper memory range will be used by the system, and thus not available to the user.  On the bright side, if you are using a video card that uses shared memory (using physical RAM for video RAM) you are still getting to use the memory that isn't available to the user, as it is being used by the video chipset. Now, if you actually need 4GB of memory (or more) in addition to what the OS uses for itself, then you probably need to use a 64-bit OS and install more than 4GB of memory.  If, like most people, you don't need more than about 2 to 3GB of memory, then a 32-bit OS is going to work just as well as a 64-bit OS.
Hello again, this is something that has come to mind after reading some more reviews for the Inspiron 1720... As far as I have seen, Vista 32-bit won't recognise (or at least has trouble recognising) 4gb of RAM and this is a bit of a concern as I intended to spend £30 on the upgrade to 4gb for the dual channel malarky.   A couple of questions here: 1) Does the Inspiron 1720 come with the 32-bit or 64-bit version of Windows Vista? 2) If 32-bit, is there any way I can get 64-bit from Dell or would I have to buy it in the shops? If there's anything else I should know about this, please feel free to add.   Thanks very much!  
Vista 64-bit, Inspiron 1720 and 4gb RAM