Shared in this case means that you have 64 mb being taken from the main memory at the moment.  The integrated graphic chip does not have 256 mb actually.  This is how I see it for your system.  The more main memory you have installed on your system, the more the integrated video chip may take, up to 224 mb, depending on what you are doing at the time.  ( A good business system, but unfortunately, Vista will suffer with this integrated chip with Aero )

 

Microsoft suggests only the Basic Vista for your system. 

http://reviews.cnet.com/4520-3672_7-6671810.html  Use this test to see if your system can handle Vista

 

 

Below was taken from the Intel web site for the 950 Graphic Core that you have for the 6400 inspiron

 

Intel Graphics Media Accelerator 950 Graphics Core

 

256-bit graphics core running at 400MHz

Up to 10.6 GB/sec memory bandwidth with DDR2 667 system memory

1.6 GPixels/sec and 1.6 GTexels/sec fill rate

Up to 224 MB maximum video memory

2048x1536 at 75 Hz maximum resolution

Dynamic Display Modes for flat-panel, wide-screen and Digital TV support