Yes, the 5770 may not quite be as fast as the 4870 in current games, but those performance differences are rather minor, a few frames per second here or there at the most common resolutions. And some games run exceptionally well on any of these cards, regardless of resolution.
What's much harder to quantify is the true difference between DirectX 10-class hardware and DirectX 11-class hardware. A great many things are encompassed under that one umbrella, DX11. For instance, Juniper has a hardware tessellation capability that games may exploit to produce much higher quality polygon meshes at fluid frame rates. Although prior-generation Radeons also had hardware tessellation, DX10 doesn't make use of it. And current GeForces have no tessellation hardware at all. At some point down the road, games will begin to use DX11 tessellation, and one of two things will happen: either the game will simply look better on the DX11-class parts, or it will run much faster on the DX11 hardware. We'll likely see similar tradeoffs with other DX11 features, including HDR texture compression and multi-threaded command processing.
Juniper offers value on other fronts, as well, including higher quality texture filtering and supersampled antialiasing, not to mention the Eyefinity feature that enables gaming across three monitors at once. Toss in much lower power draw, cooler GPU temperatures, shorter board lengths, and lighter power supply requirements, too, as 5700-series advantages over similarly priced competitors. Soon, the value meter begins to wobble in the direction of the new Radeons.
