3dmark 06 is too cpu lmited to really tell us much any more, and it is only dx9.. time to put it to pasture.
Anyway, there is no reason you cant be happy with the single 5870. More power woudl only really help you in eyefinity set ups (beyond 30" native resolution) or in games that do not yet exist.
Given that eyefinity does not yet work for crossfire, and the games that do not exist are not out yet ( ) you will be fine to only buy another 5870 next eyar when you might need it, when eyefinity works for crossfire (sometime in the 10.x drivers), and they cost less.
In my opinion, you already have one 5870--I would buy another one, since the 5870 in CrossfireX seemed to scale a bit better in some games. For the money the 5970 is only worth it if you don't currently have a 5870 to Crossfire.
Either stick with one (as Daedalus said, a single 5870 is more than sufficient!) or Crossfire your current 5870 with another one--no sense in going to a 5970 TBH.
What could possibly be your reason for adding a second 5870? Currently the only reason I can see this is if you plan to run 3 monitors in eyefinity. Otherwise it will most likely be a waste, unless you have the money and just want bragging rights. haha.
The 5970 is clocked down to 725Mhz so that it can stay in spec. It can be changed to the stock 5870 settings with the ATI voltmod utility.
In the review I posted above, they overclocked a 5970 to 900mhz IIRC, and stated they believe it will OC to 1ghz with proper cooling! That's some pretty serious power--but in all honesty, what would you ever use it for (except to say "I have it").
Will be a minimum of 6 months before there are even games available that will tax this card properly. To be honest, the games out now are not even really a fair benchmark of the 5970's performance seeing as how the DX11 titles will be where it really shines..