Even if you're comparing your mining return to interest at a bank (which, admittedly, is quite low), it's still silly to buy cards that won't pay off for 9 or more months.
Besides the usual argument, "Bitcoin could be $3 in a few months!" which no one can really refute...
What if the 7000 series cards have a much better stream processor count? Another thread mentioned them having *double* the stream processors. That's only a rumor, of course, but what if something like that happened? Does any of us know what the 7000 will bring? Isn't it prudent to wait until the details have been released, before sinking one's savings into 6990s?
Those who *didn't* buy cards with a long payout will be in a better position, because they can buy the 7000 series cards and keep up with the difficulty increases that will no doubt ensue.
Those already "invested" in $500 5970's and 6990s will have to watch their returns diminish, and won't have as much capital (or free slots) for 7000 series cards as everyone else.
So there are many things to consider when you go "all in" at this point.
Speaking as the owner of 2 6990s, I'd like to point out that no matter the power of the 7XXX series cards, the rigs we have still retain value.
For example; lets say the 7XXX series cards come out, and they do have 2* the stream processor count. This should mean they are
more expensive than the previous generation (because of these improvements). With the 6990 topping
$700, I would find it hard to see anyone other than extreme enthusiasts buying these cards. This is one of the two problems for the new 7XXX series of cards.
With this cost, it's
very unlikely most pre-built manufacturers (everyone from ibuypower & newegg types, to HP and DELL) would see these cards as a viable option for their machines.
The second problem is the bulk purchase of the older generation of cards. When a large manufacturer like HP, or DELL etc.. Build a line of computers they buy / order the items in bulk. This means they can only afford to integrate the
latest and greatest into their lines
very slowly, especially when the cost is so high.
This means for a decent amount of time the old 6XXX series of cards are still the best you can get without huge financing & building it yourself. The effect of this is that I can still sell my computer for a more than reasonable price even after the 7XXX series has been released, because no-one will be able to; afford / get them in a pre-built.