is the greatest
At current, ATI names each card by generation, series, and by performance. The first number is the generation number, for example, 5000, and this is related to the chipset used by the video card. The second number indicates the series quality in the generation, starting from 0400 to 0600 at entry level, for media and home theatre, 0700 for low intensity video games (typically using older graphics engines, or widespread games, such as Starcraft and World of Warcraft) or high-intensity games with lowered settings, and 0800 for high-intensity games, such as Crysis or Far Cry 2. ... The third digit is the relative quality, within a series- a 5850 is less powerful than a 5870.
So while a 5000 series card is (per the Wiki chart) a mainstream card and the 4000 series is Budget, the 800 series is suitable for high-end gaming, where the 400 is suitable for entry-level video needs.