The difference is negligible in terms of the processor itself in gaming or not. However the 930 has more pci lanes and can support dual x16 SLI/Crossfire, or more! Also, in the future you could upgrade to a 6-core cpu, but as far as I know any cpu's that aren't currently out won't be on either platform (1366 or 1156).
If you can afford the stuff that will make the 930 better (expensive Mobo, extra GPU, triple channel ram) go for it! It'll give you better performance, you'll have fun OCing it, it'll look good, everything you ever wanted. But there's really no guarantee that in the future a few new parts will give you any real performance boost, so the time until you do a complete rebuild may not be too big of a time difference (it's hard to say though)
If your trying to save a few bucks, go with the 860, 1 GPU , the dual channel ram, and you'll still rock out the latest greatest games for quite a while.
The i7 870's have dropped in price and are alsmost the same as the i7 860, so I would definitely go for the i7 870. It has a better turbo boost when compared to both the i7 860 and the i7 930. At max turbo it will run at around 3.6 GHz and at min turbo it will run at aorund 3.2 GHz. So unless u really want to do SLI/Crossfire and have triple channel memory, I'd say the i7 870 would be the best choice.
You wont need an i7 actually as there is no use of hyper threading in games.. You'll be nicely served by an i5 760 or a Phenom II X4 955 BE (in case you may think about going the AMD route)..
You wont notice a differance between the P55 and X58. Unless you're so happening to run on tri or quad sli/CF gpu setup (quad setup as in four gtx 480's and not as in two hd 5970) which is the only way that you would see a difference. (x8 and x16 have no major performance drops as seen in this review http://www.tomshardware.com/reviews/pcie-geforce-gtx-48...)
And for all the core i7's. It's overkill. As Emperus said, if sticking with intel, The core i5 is all you really need for gaming as mass majority of them dont even use the HT.
Save the money by getting core i5 and put it towards better graphic setup. A better graphic setup will give far better performance that a faster/more multi-threaded cpu.
Is it good to buy new parts in a few years to upgrade a x58 setup or should I get a p55 setup and just do a total upgrade in the future?
As for this, if you're planing to upgrade in a few years... well even the x58 sometime next year will be replace from what i heard. So kinda a pointless as you'll need to upgrade every thing (even the x58) next time you'll change setups.