Lol don't listen to these dudes, I've been playing on a Dual core and NVIDIA 8800 GT ( LOWEST POSSIBLE GPU REC FOR BF3) and been playing fine (minimum of 35 fps, I had 55-60 stable for hrs) on minimum settings and spotting like a 1 man army for the whole team on maps like project fire storm. I'm upgrading to a GEFORCE 670 for a huge performance increase but I'm telling you, all you need is skill, don't need any fancy sh$t to run games.
There's one issue: 8800 GT is much more powerful than HD 6450.
Do you know if how does the hd 6750 compare to the hd 6450?
HD 6750 is much more powerful, around 3.4 times. But it will not work on your power supply unit.