If you're going Intel, you can go for the baby I3. If you're going AMD you can probably save a bit of cash and pick up a decent quad core which will run the game fine.
Really for BF3 it's all about the graphics card, which while people have been saying ATI (and normally I would as well) I would be tempted to go for Nvidia just coz they run BF3 better. The problem is with Nvidia is that they don't really have a decent low end card, whereas with ATI you can pick up a 6770/7750 for about R1200 which will run the game at playable frame rates.
Really for BF3 it's all about the graphics card, which while people have been saying ATI (and normally I would as well) I would be tempted to go for Nvidia just coz they run BF3 better. The problem is with Nvidia is that they don't really have a decent low end card, whereas with ATI you can pick up a 6770/7750 for about R1200 which will run the game at playable frame rates.