If you mean 6-screen eyefinity you'll need an eyefinity 6 edition card. Is that what the game edition is?
Also if you want to run 6 screens you definitely want to get a 2gb card for the memory, or the 4gb 6990. I wouldn't recommend crossfiring the 5870 for 6 screens as the main bottleneck is the 1GB memory on it and having 2 cards won't make any difference to that.
So yeah, get a 2gb single card or the 4gb 6990.