1- Both nVidia and ATI OC very well just depends of ur skills and the components.
2- Single: ATI 5870 Dual (SLI/Crossfire): ATI 5870
3- The performance difference isn't big but in this case I prefer CUDA and GTX480
4- What other components do u have? The 5850 is more cheap and have a good performance.
PsychoSaysDie has posted some benchmark which tell quite different thing seeing that Radeon HD5870 is better then in GTX 480 in single configuration and GTX480 phases out Radeon HD 5870..........do u think this benchmark is right..?
I think Asus's ROG Radeon HD 5870 will better since it has 2GB of mem and is of same price as GTX 480 which has 1536 Mb of Memory buffer............
Thanks for your help............