Then, the 7850 seems to be my best choice. But I read it has some compatibility issues because it was released recently and the drivers are not very stable yet, it makes me doubt about getting it or not.
Yes the 7850 lies somewhere between the 560Ti and 560Ti 448.
The 7850 occasionally evens out with the 570 and usually loses by a few FPS (~5 FPS). Not an issue that Overclocking can't fix.
The real gain is really on the efficiency side. The 7850 provide an 'almost GTX570 like' performance whilst being ~$60 cheaper, runs cooler and consume way less power. IMHO better save the money and get the 7850. It'll cost the same as the 560Ti 448 when you order a separate BF3 but you're getting a card with far superior efficiency. And, you'd be able to save on other thing like the PSU for example. With the very low power draw, you could drop the PSU to 550/600W and save money or stick with the Corsair TX650 without the need of upgrading it should you wish to get a second 7850 and CF them.
Overclocking GPUs are fairly easy with apps like the MSI Afterburner. It's the same thing as overclocking your CPU (find highest clock speed at certain voltages and test for stability) but much less time consuming since you won't need to reboot and enter the settings in the BIOS. The 7850 OC should play well against a stock 570 but bare in mind it won't be able to compete with an overclocked 570 when both has the same percentage of performance gain. But even a stock 570 would run today's games at high - ultra settings so who cares really when you're able to get 50-60FPS constant with your overclocked card
Also, nvidia has the physx feature, which AMD doesn't....
And the crossfire possibility with 2GB VRAM seems more future-proof on paper because it leads to more FPS, but I'm not planning to do it because of micro-shuttering and compatibility issues, so I will go single card.
I read BF3 needs more than 1GB VRAM on ultra settings, so I'm a bit worried about 1.25GB being too little for the games that will be released during the next year.
If you play at 1080p resolution 2 GB is a waste. If you want multiple monitors or higher res than 1080p then a 7850 wouldn't perform well anyway. I really believe that 2 GB in such cards is a gimmick.
edit: wrote 7870 instead of 7850