OK. First, since you have been planning to play Crysis when it comes out, it is suggest that you buy Windows Vista and the NVidia GeForce 8000 series video card.
Because if you don't you can not enable the maximum graphical settings because it uses Direct X10, in which Vista and the 8000 Series support. Both are needed.
As for Video Card memory goes, how high of a resolution are you planning to play the game? Video card memory only affects how the video card will slow down the higher the resolution the game gets.
Let's just say we have two identical video cards, except one has 512MB of memory and the other one has 256MB. At lower resolutions (1024x768) , the performance difference most often don't go beyond 2 to 3 FPS. (Say 117FPS and 120FPS) However, when going to 2048x1536, then more likely, the game won't work, or will run 5 FPS on the 256MB one, while the 512MB one will run at about 13FPS.
If you only play at 1024x768 or lower, then you do not need to worry about video card memory. What's more important is what is the video card is, a GeForce 8600 GT is MUCH faster than a 7600GT. Remember that.