I think those figures would be reconciled by considering whether the OS is 32bit or 64bit.  If you have WinXP 32bit, with 4Gb physical RAM installed, the system info will read perhaps 3.5Gb RAM  (4Gb minus the video card's half Gb onboard memory).

     If you have Win7-64bit, with 8Gb physical RAM installed, the system info will show 8Gb available;  the video card memory would not be deducted from the main RAM capacity, unless the mobo/chipset has an inherent addressable-memory limitation of 8Gb.

     Some mobos will not support a 64bit OS.

     Just my judgement, but...  budget permitting, I would not buy a 32bit machine for 3D rendering.  A 64bit machine's greater memory capacity will allow for much more complex scenes, more sophisticated materials, higher render quality settings (such as more ray trace bounces for IDL), and larger pixel dimensions.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5