Self-build computer, i7 or phenom ii x6?

Hi, I am planning to build a new pc to replace my old system. The old system is a dell pentium 4 with 256 megs ram, integrated graphics and an 80gb hdd. Anything I build will be a consderable step up. I was comparing prices of cpu's, trying to discern the best value for money and wondering are there any noticible differences between the intel i7 860 and the AMD phenom ii x6 1055t, and if so is it worth the extra cost.