This game is optimized very well. I know every case can be different, but on my 6 yeal old, 32-bit system, with "4GB" RAM and Radeon 4870HD its running smoothly and perfectly so far. Might want to check some stuff on your system if its lagging.
Also OP like others said, it looks like your GPU is very much under powered and too old for this. There is a limit to how low a bar they can set and still retain some quality. Same as with newer games not working on older OS's