Video memory problem

I have a problem with my video card during games that require a higher video memory.
I have a Nvidia 9800 GT video card and 4 GB RAM on my computer but when I games that have higher graphic I get an error message from most of them saying that I don’t have enough Video memory.
I am using Vista 64 bit version.I checked the video memory for my card and it has 1GB but it also takes part of the RAM memory from my pc so when I check it with DirectX diagnostic tool under the Display tab it shows “Approx. Total Memory: 2801 MB”. I know that video cards share memory RAM when needed to but it seems mine is sharing it all the time and therefore I don’t have any RAM memory for when I play the games ergo the error message i receive.
My question is if anyone knows anyway of stopping this memory sharing option or if I can reduce the number of shared MB from the current 2801MB to 1500MB for example and see if I still get errors in games.
Thank you in advance for your reply.

You should post this in one of the gaming-related forums. This one is about GPU computing and I think not a lot of us reading this forum are gaming.

You should upgrade your PC according to the configuration required by that game