You should be OK on medium. Just make sure you turn all AA/motion blur/HBAO stuff off. I'de bet you'de be consistantly over 30fps there. My old 5670 with a 2500k was running on high around 20-30 fps. Enjoy the game!
well, I would think that is underpowered for BF3, although I have a much beefier setup and play and high/ultra. One thing I can say is that it will probably really struggle with large multiplayer maps. I've seen benchmarks between the i5 quad cores vs i3 dual cores and the i3 was maxed out, dropping half the frames that the bigger i5 could do. I'm also sure the FPS was too low to play.