I would be tempted to sell your GPU and go for a better single card. The 1GB vRAM might become somewhat of a bottleneck in future. With overclocking, a GTX 660Ti should beat those cards in SLI and it has 2GB of vRAM. The GTX 560Ti would cost $200 anyway and you could sell yours for ~$100. The GTX 660Ti is $300.
it's kinda iffy keeping the HD the way it is going to another mobo and all. it may work. Friend did one over the weekend. booted to desktop had to play phone tag with Microsoft but it worked. machine loaded necessary drivers..... keep your fingers crossed.
agree with the 560. adding 2 won't do anything to make BF3 more playable. you'll still be limited by the cards architecture and still only have 1gig of usable vram between the 2. not much of an upgrade.