Not always but it's a good idea but not always the best deal and value.
i can see it like this if you are buying this year. 7970 in cf = 1100+++$
a 7990 at launch should be 850- 1000$. Overclock versions might come out later.
a 7970 clock = 950- 1ghz + lowest price 550
a 7950 clock = 800 - 900 lowest price 450
a 7990s clocks shouldbe dual 850 mhz. priced at around 850
7990 still would be best value. i dunno whats gonna come after the 7990, personally id settle with a 7990 for convenience sakes.
The best of the best is only going to come out after the 7990 when the release a dual ghz card.