I think it doesn't matter what brand the cards are. Deep down they're both the same.
I think you should wait for Ivy Bridge. Maybe it will not be worth the extra price but when it comes out prices on Sandy Bridge will drop so you could get a better deal
As long as the two cards share the same model number (in this case HD6950), you can use them in Crossfire. In some cases, they do not even need to be the same model. In your case, the HD6950 can be used in Crossfire with the HD6990, HD6970, or HD6950. Note that any card greater than the HD6950 will down-clock to HD6950 speeds.
As for your new laptop, wait until Fall to see what's available.