XPS 8000 - I7-860 - 8g ram not detected


Oct 22, 2011
Hi everybody,
I have a Dell XPS 8000,DP55M01 motherboard, I5-750, Corsair 650W PS,4x2gig RAM 1066,GTX 260 1796mb, Win7 64. I just bought a I7-860 cpu to upgrade my old one but as soon as I put this cpu on my motherboard, I just see 4gig of ram and I have done all the test that I can do. My bios tells me that I have 4gig of ram and if I try with only 4 gig the bios tells me that I have 2 gig. When I switch back to my I5-750 everything is ok. What is happening ... normaly those 2 cpu should be compatible. Is there anyone who can help me with this??