It is provided that the second card is cheaper than the first and there is a connector for the bridge. If there is not then avoid going crossfire all together with these cards. Avoid the purchase of the second card and upgrade the main card instead. pci-e just isn't fast enough for good crossfire/sli performance without the bridge.
Will CFing two 5570s increase performance, the short answer is YES. However, given that this is really a general purpose GPU, it won't make the 5570s into a gaming platform. If you expect a radical increase for gaming, you will be disappointed.
The question is this, what are trying to increase performance for? Also, is the rest of your system, particularly your power supply, up to CF requirements?