HardOCP has compared ten Radeon HD 4800 and GeForce GTX 200 series graphics cards to see which video cards offer the best value:
We look at the 10 combined Radeon 4800 and GeForce 200 series configurations. Our evaluation scales from what you get when you spend $150 on a video card, to what gaming gains should be expected when you spend $1100 on 4870X2 CrossFireX . Real world gameplay and Apples-to-Apples as well.
Check it out over here. Overall they found that the Radeon HD 4850 in CrossFire offers a lot of value for your money. The Radeon HD 4870 isn't really worth it now that NVIDIA dropped prices on its GeForce GTX 280 and GeForce GTX 260 and the Radeon HD 4870 X2 is too expensive for the real-world performance level it provides.