InsideHW.com has reviewed PowerColor HD4890 graphics card and compare its performances with Radeon HD4870 A quote from the article:

Last few months rumors about new redesigned ATI's GPU that should replace HD4870 were appearing on regular bases. Reason is simple: price for GeForce GTX260 has dropped significantly so, logically, HD4870 has to follow the same trend to keep up the pace. Of course when something like this happens, very often "a gap" in product lineup is created because price for HD4870X2 hasn't change significantly. Since ATI's single GPU high-end graphics card operates at relatively high 750MHz, it is obvious that without redesign in GPU architecture higher frequencies and performance levels cannot be achieved. ATI tackled this problem differently from its previous practice.

>> PowerColor HD4890 1GB Review