Well depends. The question is can you step up again after your first step up? for instance, if you say upgrade to the ULTRA version, would you be able to step up again for the next gen?
if yes, then i would say then go for it assuming that the next gen of geforce arrives within the next 90 days. This way you can maintain the most amount of your money in your pocket. IF you get the Ultra now, you may be able to refresh the step-up time frame and if the next gen comes in that period, you can again step up to an equivalent card by paying less. Rather than buying a whole new card for the price of the first one!
How about you put that $100 on your savings account, wait a few months for the 9 series. I'm sure you will agree when I say that no one can tell the difference between an ultra and a gtx when it comes to fps.