i have bought assassins creed and i dont have the recommed system. I am thinking about to buy a new and better video card. but it also say that my CPU's frequency to low is (1.9Ghz). But i have Pentium 4 3.0 Ghz single core. Does the game work on my computer if i buy a new video card??

Some users are running the game with single core CPU's like yours, but the main reason is usually because those who can run the game have good video cards, and by that, I mean well above the minimum requirements of Geforce 6800 0r Radeon X1600.
Personally, I wouldn't try it on a single core without at least a video card around the performance level of a Geforce 7800 or an Radeon X1950