Crossfire and SLI won't usually make your graphics performance twice as fast. But, for a highly popular, well-optimized game like BF3 you may get pretty close to twice the performance. For one thing, both cards have to load the same data into their RAM. So, you're still effectively limited to 2 GB of RAM. Beyond that, I don't really understand the technical details of the bus structure or how data is synced between the two cards via the Crossfire bridge, but there are inefficiencies and bottlenecks that will keep you from getting twice the performance.
Nonetheless, you will see increased performance in BF3 in Crossfire and I personally think it's worth it, if the price is right. Just to give you some idea of what kind of performance bump you'll see, I ran a quick benchmark on BF3 using one 1GB 5870 and then using 2x 1 GB 5870s in Crossfire. Note that these benchmarks are at 4800x900 since I can't get smooth, acceptable framerates in 32+ player matches if I go any higher. You'll probably get better performance at higher resolutions, since you're on the 2 GB model and I understand that BF3 is a memory hog. I use medium textures and shadows, 8X anisotropic filtering, and everything else at low.
I ran both of these benchmarks on the same server playing 32 player deathmatch on Noshar Canals.
FPS when running from one card:
Min: 34
Max: 63
Average: 45.9
FPS when running in Crossfire:
Min: 63
Max: 102
Average: 80.2
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