You can set RAM to run at different mhz in your BIOS (if you unlock the right things). So you can change it to run at DDR3-1333mhz (or even lower lol). With that processor you can OC it to 3ghz, and that should be enough to play BF3 on. Then when you want to upgrade the motherboard, you can push the RAM up and change the FBS on your processor to run at a higher speed (or the same, but you will still have to change your FSB if you speed up the RAM).

You can pick up 4gb of DDR3 Corsair RAM (Vengeance, very good) on newegg for $40
http://www.newegg.com/Product/Product.a ... 6820233141

Plus Rabbits 2 cards, that will run you just over $200 for a tidy upgrade.