I was only recommending those parts because they were recommended to me in my thread
here. But I don't mean to double post either, I just felt the tip so worth (i5 instead of i7) that I forwarded it. But still I am not sure about the advantages btw P55 vs X58, i5 vs i7, the obvious disadvantage is price. I will have a deeper look into this this weekend.
As stated, I misread your post, sorry.
The P55 is limited to 16 lanes of PCI-Express, so running multiple graphics cards will be limited to 8x/8x, and you have to watch the boards closely because some only run the second card at 4x. Not a limiting factor to some, but to me, with a $2000 budget, the i7 920 and an X58 board is a no brainer.
A couple other differences:
P55 boards run Dual Channel memory, X58 runs Triple Channel.
Core i5 cpu's don't hyperthread. Core i7 860 fits the P55, and it hyperthreads, but it's the same price as the i7 920, so, no brainer again.