well in my opinion depending how much you want to commit to bitcoin just stick with the 2 cards for now and wait untill we find out whats happening with the ASIC miners
I think we know what's happening with the ASIC miners.
If it weren't for the corresponding skyrocketing exchange rate as well, nobody would be talking about adding more GPUs.
Difficulty is going up at least 10% with this next adjustment then we get to see all of the Avalons kick in, and ASICMINER continuing its assault.
Unless there's a good place to use these GPUs after a few months, buying more GPUs today is probably not a good idea.