The motherboard has completely different sets of slots for memory and for video (and other adapter) cards. A single video card takes only one slot no matter how much memory it has (although some cards are physically so large that they prevent a card from being inserted into an adjacent slot, so they effectively take up two slots). But the memory slots are in a different location of the motherboard so they're not affected.
Note that if you use a 32-bit OS and 4GB or more of RAM then (a) the OS will only be able to access the 1st 4GB of RAM and (b) the memory in the video card replaces some of the RAM so you won't be able to use all of it. None of this has anything to do with the physical slots that the memory or video cards go into.