Windows 32 Bit will set aside up to 750MB of RAM for use for devices. This RAM cant be allocated for anything else. This RAM is not counted in the total amount of RAM used by the system nor is it accounted for in the resources. When you look at your system properties you will see that Windows Vista and Windows 7 will report that 4GB are installed, but only 3.25 is usable.
You can in fact increase this amount but it will remove from the total amount of RAM available for the system to use.
If you have a 64 Bit OS installed, VRAM is not counted at all.
I have 6GB of physical RAM installed. I also have 768 MB of RAM on my video card and my video card is working at 70% load at this time. Yet it doesnt show up as part of the resources being used. But that is only cause my system isnt limited by the amount of RAM that can be used. If I was on a 32 bit OS, I would lose over a full GB worth of RAM that would be dedicated to my hardware cause 768MB is on my GFX card alone.
So yes you have a very real potential that you will have less than 3GB of RAM available for use with your system. Take out the GB for the new Video card and add in up to an additional 750MB for other hardware and you could be down to 2.25GB - 2.75GB Usable. c0rr0sive is right with what he said.