I would suspect that it has something to do with CPU performance. BF3 takes advantage of 4 cores. Also, you run into issues with windows as you knew you made notation of it as you are memory capped. A quad core would be more advantageous for BF3.
The thing is , i have been playing from 1 year and fps have never dropped below 30 . And i am not playing the game at ultra but at high setting . Aftersometime i dont know why fps goes to below 30 from 2 days .