i don't recommend crossfiring 2 5670s, at all, 1 5770 is faster if possible get 1 5850 / 6850 but if you must then just pop it in if it didn't come with a crossfire bridge or any spots to put one then its running off software crossfire, just set up the drivers and your good
2x HD5670 will make a good performance but still, I suggest you to use a single powerful card, single card will gives you less problem, less heat, less noise, less power consumption.
Basically HD5670 is a low end gaming card. May I know the rest of your specs? CPU? mobo? RAM? PSU? Monitor?