Radeon HD 4860 is a high-end GPU based on the 55nm, third unified shader architecture, R700.
It's based on the RV790 GT Core (related to Radeon HD 4830) and offers 640 Shader Processing Units, 32 TMUs and 16 ROPs a 256-bit memory interface of fast GDDR5. The central unit runs at 700MHz and the memory clock runs at up to 750MHz.
It's expected to perform only slightly better than Radeon HD 4850 as it comes higher clocked and paired with fast GDDR5 but features less Shading Units and less TMUs. It should perform considerably better at higher resolutions but overall, expect only a 2-3% performance boost compared to Radeon HD 4850.
Some of today's modern games should run fluently at high settings but with modest resolutions. DirectX 11 games aren't supported.