Are you running/planning 32 bit or 64 Bit operating system and how much memory is installed on GPU card.
With 32 bit Operating system you will see all 4 gigs, HOWEVER only 3 -> 3.5 gigs will be available to operating system and programs depending on amount of viedo memory. That said, I always use the 4 gigs vs 3 gig of Ram - One desktop vista 32 and two laptops (1 vista 32 bit & 1 Win 7 32 bit).
With 64 bit (or planning on upgrading to 64 bit), deffinently go with the 4 gigs of ram
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