Mobility Radeon HD 5870 Crossfire is a solution of two Mobility Radeon HD 5870 put together using AMD'S Crossfire technology.
Check the page of Mobility Radeon HD 5870 to know more about its chip.
Crossfire relies a lot on proper driver support and may suffer from micro-stuttering in lower frame rates (below 30). Benchmarks indicate the performance is overall, is up to twice as better as a single Mobility Radeon HD 5870 Crossfire performing by itself but at times (depending whether or not the 3D game supports crossfire or in the graphics driver) it performed worse than a single Mobility Radeon HD 5870 Crossfire. This combination might consume up to 100 Watt though the average power consumption might be slightly lower.
Benchmarks indicate today's modern games can be played at high settings using a 720p+ resolution, assuming the game supports Crossfire, properly.