Yes, it could, unless you're running a 64-bit OS and processor. Most computers, which are 32-bit, have a total or 4 GB of addressable memory space, which includes video memory, sound card memory (if you actually still use one) and system RAM. Therefore, if you put in a 2GB video card, you can't make use more than 2 GB of system RAM.
That's not entirely accurate. PAE makes it possible to address 4+ GB of addresses on a 32-bit system, so long as your operating system supports it fully. AFAIK both Linux and BSD happily do, and certain Windows versions also support it.