AMD Radeon HD 5870 Scaling on Intel X58 vs P55 Chipset
 

For those of us who are interested in power consumption, heat, noise, and not all that worried about a 2% to 7% difference in the benchmarks, then the Lynnfield platform is an attractive alternative with the latest generation GPUs. In fact, without the 920/X58 hanging around at similar pricing, the general thoughts/concerns regarding 860/P55 would probably be significantly different within the enthusiast community. The 860/P55 is a very good platform, especially for those running at stock or near stock speeds where the aggressive turbo mode will make a difference in daily computing tasks and your pocketbook.

That said, if you are running a single card such as the HD 5870, either platform is fine.

http://www.anandtech.com/mb/showdoc.aspx?i=3649