The R7xx family from ATI is slated for arrival this Spring and will apparently be a bit different from other generations of graphics cards. Ever since early 2006, the story has been that ATI will put the one billion transistor chip aside and instead go for a much more modular GPU, where one core could be used for the low-end cards. Two cores for the mid-range cards, and four to satisfy the high-end users. This could throw more than a few thumb rules out of whack though.
First of all, this could very well mean that the rumors of R7xx being made with a 45 nm process are true (translated source). Usually a high-end chip is made with the same process as the older mid-range GPU, and it’s a lot easier to move from 65 nm to 55 nm than from 55 nm to 45 nm.
Disregarding the previous statement that rule of thumbs may not apply here, we can always do some graphics card generation math. Usually the high-end of the current generation performs like the next generation mid-range GPU. Knowing that the current high-end will be two RV670 chips it would be feasible to believe that a single R7xx core would offer performance much like the current RV670. The high-end card would then offer the theoretical performance of four times the Radeon HD 3870, if clocks are somewhat similar and multi-core scaling works well.
Two different sources (both from Taiwan) claim two very different things here. The first one confirms the above paragraph and said that the single R7xx chip would perform slightly better than the RV670. The low-end would then perform much like the Radeon HD 3800 series.
The second says that the single chip will quite different and actually be much, much smaller than the RV670. Only a 300 million transistor chip with a total surface area of only 72 mm². This brings us back the rule of thumb being out of whack. Slides are MIA, and until they appear, maybe you should stay on the sure side of things and wait for more reliable information.