It would be a waste of money to buy a 560. A 560 is marginally better than the 6850.
If you have another pci e x16/x8 slot,you could buy another 6850 and crossfire. If not,save up for a HD 7850 or wait for the gtx 660.
since you already have one 6850 1gb it doesnt worth to spent 220 euro to go for the gtx 560 TI because gtx 560 ti cant play BF3 on ultra at 1080p. it is better to save money and go for hd 7870 oc for 300 euros or wait for gtx 660 ti. to help us to give you more accurate answers pls tell us your system specs and your resolution.