The Most Comprehensive AMD Radeon Linux Graphics Comparison
Radeon HD 4830: An RV770LE is found inside the Radeon HD 4830. She is running with a 575MHz core and 900MHz of 512MB GDDR3 video memory.
Radeon HD 4850: This Diamond Radeon HD 4850 512MB graphics card is one of the engineering samples received in advance of the RV770 launch. She runs with a 625MHz core and 512MB of GDDR3 memory. Besides timely Catalyst support with the Radeon HD 4800 series launch and going forward, the Radeon HD 4850 worked on the open-source driver at launch. This though was largely an exception and has not become a trend.
With the xf86-video-ati and Radeon DRM using AtomBIOS to program the graphics card, it's less of a burden enabling initial hardware support compared to banging the hardware directly, and in the case of the Radeon HD 4850 it was easy bringing up mode-setting after the Radeon HD 3000 series was supported. However, it took much longer for 3D acceleration support to arrive. It took long even for 2D acceleration to be supported, that came in January 2009. Major hardware launches since the RV770 have usually not been trouble-free, but requiring at least one or two kernel release cycles after the hardware has launched before there is mainline support. The time required for 3D support varies, but with the Radeon HD 6900 "Cayman" series, it took a couple of months for 2D/3D/X-Video as an example.