Radeon HD 4870 will work at 750MHz. This is the final clock and we believe that most of the reference cards will work at this particular speed.
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This is a 25MHz slower speed than Radeon HD 3870 but the card will have more Shaders and more raw performance. It is, of course, a dual-slot card and we’ve already showed you how it looks here.

The slower of two, Radeon HD 4850 will work at 625MHz compared to Radeon HD 3850 that works at 670MHz. This one also uses GDDR3 memory, while the Radeon HD 4870 uses GDDR5, both 256-bit.

If ATI wanted to get rid of the XT, PRO and GT suffixes we still wonder why this HD prefix survived; probably because HD sounds cool.