With the recent rise in BTC value I'm thinking of adding another 7970 to my rig.
Taking BFL at their word, in about 90 days difficulty will be skyrocketing due to their shipping of ASIC models. In less than thirty days after that (early December) the block reward drops in half to 25 BTC per block.
Just out of curiosity, since you decided to add capacity using GPU is this because you still think you can mine profitably by the end of the year, or do you plan to just mine on it while the profits are possible and then sell it without taking too much of a hit since there still is demand for it?
(Or, are you betting ASICs really are further out and won't see them in 2012, perhaps?)
Honestly everything with BTC is a huge gamble. If I was to buy 20 btc today for ~$200 USD who knows if it'll be worth $50 or $500 at the end of the year.
If I spend some money on GPU's now, who knows if they will be profitable for another 1-2 months or even for another year (that would assume BFL doesnt come through and price of a BTC increases enough partially offset the halving).
It is this uncertainty that is actually kind of fun about the whole gig. It is similar to investing in riskier stocks that can either burn you or reward you for the risk.
At least, if nothing else, at the end of the day a new 7970 is going to still be worth quite a bit of money in 2-6 months.