When looked at from a pure performance standpoint, then the Sapphire HD 4850 X2 is among the top cards on the market. It beats the NVIDIA GeForce GTX 280 by about 3%. What is also very interesting is that in higher resolutions the HD 4850 X2 can also beat the bigger brother, the HD 4870 X2. The exact cause of this is unknown, but my educated guess is that it has to do with the slower timings of the GDDR5 memory on the HD 4870 X2. As you can see in our benchmarks the natural habitat for the HD 4850 X2 starts at 1920x1200 and up. If you plan to run lower resolutions then your money is certainly invested better elsewhere.
While NVIDIA has a single GPU design with the GTX 280, AMD has chosen to use a dual-GPU approach for their highest-end cards. For you this means that you have to accept several limitations of this technology. One is that ATI has to create a CrossFire profile for every single game to benefit from their dual-GPU technology. Another factor to consider is that CrossFire does not work in windowed 3D. So if you play any windowed games for whatever reason then the HD 4850 X2 will perform exactly like a HD 4850 single card.
I found it surprising to see such a nice overclocking headroom on the HD 4850 X2. Once you successfully master the process of getting the card to run at matching clock speeds, you can achieve some nice performance gains; in our case about 15%. Sapphire's implementation of the HD 4850 X2 is flawless with one exception. The fan is disturbingly noisy in both idle and load. Actually, it is the noisiest card I ever had in my hands and under no circumstances would I accept that amount of noise in my every day work PC. Even though recent Catalyst Control Center versions add fan control to the Overdrive tab, the lowest fan speed you can select there is 20%, which is over 50% faster than the typical idle fan speed of the HD 4850 X2 (12%). The only way that I see to quieten the card down some is to modify the BIOS using RBE, but that is a somewhat complex procedure and not for everybody.
With a price point of $419, the HD 4850 X2 is actually more affordable than the HD 4870 X2 and the GeForce GTX 280. I expect another round of price drops by NVIDIA soon, to counter Sapphire's new offering. Overall the Sapphire HD 4850 X2 is a good card if you can stand the noise (which I doubt you can).