Another missing memory question

ive got an MSI mother board (MSI P7N SLI Platinum ) and i am running a pair of XFX video cards. (
XFX PVT84JYDF3 GeForce 8600 GT 512MB)

my OS is Windows XP 32-bit and ive got two 2-gig Ram modules installed.

im familiar with the concept of hardware demanding memory addressess and how windows can only use what ram is offered to it by the system bios but something is going on with my PC that i dont quite get.

with both of my video cards installed, running in SLI or not, my available system memory is 2.00 gigs. however, if i take out one of the 512 meg cards my system memory will jump to 3.00 gigs.

i think in either case i should have more ram available but why the full 1 gig jump from removing a single 512 meg video card?

also, would an incorrect VGA BIOS on one or both of the video cards be altering how much system ram needs to be used by the card(s) my VGA BIOS is suspect but i cant find anything at all for my specific model of video card. so i dont really know if it's correct.