basicly you will get better frame rates on the GTX 560. image quality will depend on the settings you have the game set on. but with the 550 you will have to lower the settings to get the same frame rate as the 560. hope this helps
here are the reviews of both the 560 and the 550 Ti. Even though the cards are not directly compared to each other, you can plainly see the 560 trump the 460 1GB by a fair margin, while the 550 Ti regularly falls short of the 460 768Mb.
What resolution? If you're wanting to do something like 1920x1080 with most of the eye candy on, I imagine those GPU's are going to get swamped pretty quick. Here's a great comparison across that range using DX11 (like those Battlefield titles):
The rest of the comparisons are great resource as well.
There are two 560 (non-Ti) in there...one of which is sold @ Newegg and it's solid one from a build standpoint. And the MSI Twin Frozr II is OC'd to 870 MHz compared to 810 MHz on the Asus you linked. But there are plenty of AMD 6850's and 6870's performing around it, depending on the resolution, so I wouldn't count them out either.
Ultimately, I think a 550 Ti is going to get swallowed hole by these FPS's. I ran my i5's HD 3000 graphics OC'd while I saved for my MSI 560 TI HAWK and running BFBC2 like that with DX9 forced was definitely passable @ 1280x800, but when I could up the res, max out the detail, AND up my frame rates, it really did help my gameplay in multiplayer A LOT.
If BF3 is your main goal, I'd personally recommend holding off, while continuing to save some $$$ until at least the beta comes out in September and see where performance is then. Undoubtedly, the internet will be flooded with benchmarks then.