Is a high-end 6000 series (6800XT) better or worse than a low/mid-range 7000 series (7300GS). They both have 256MB onboard.
The high-end x8xx or x9xx cards are always a lot better than the low-end x2xx or x3xx cards of that same series. The low-end cards are budget cards, wich are usually bought by ppl who dont wish to play games, or dont have very much money and have to play older games.
Sometimes the gap beetween a series is so big, that a high-end card of the last series is abt the same as the low-end card of the current series, in overall performance.
The high-end still has a better core, having more pipelines, but it usually uses a higher process, and so the core clock can only get to a certain clock speed, while the new low-end has less pipelines but can achieve higher clocks, and in wich case can have similar performance.
With graphics card, the best way to know how well a card will perform and fast it is, is to look at the average price. Two cards, both priced ~200$, will give USUALLY guarantee u very similar performance.
The 6800 XT is the slowest of the 6800 family (the fastest being the 6800 Ultra Extreme). The 7300 GS, a low-end card of the 7 series has about the same core performance as the 6800 XT, purely because of it's higher clock speeds.
In this case, the 6800 XT is still a lot better, because it has a 256-bit memory interface, while the 7300 GS has a 64-bit memory interface, despite the fact that, again, the 7300 GS can achieve a higher memory clock.
The simple answer is, its possible for a high-end card of a former series to have similar performance as a low-end card of the current series because of cheaper technology and new ways of giving more performance for less. But the high-end card almost always have places where they will always be better, and so to know how well a card will perform, check it's price.
Or better yet, use this site to tell u exactly how good each card is -
http://www.gpureview.com/show_cards.php?card1=441&card2=376
