P55 chipset if for the core i5/i7 processor and the 1156 socket. X58 is for core i7 and the 1366 socket. CPU performance wise they are very similar, X58 is the more extreme and has the most PCIe16 lanes for gaming. You can easily spend 100$ more on the X58. Also X58 is meant for triple channel DDR3 while P55 uses dual channel, but you can still use two sticks of memory with the X58.
Getting the i920 and a X58 board is still a good way to go. The new 860 is very fast with a P55 MB.
You can connect as many fans as you want to the PSU, a lot of fans also have a molex connector that can be stacked.
For the RAM read the reviews on newegg.
If you want a more budget minded build that would still be fast go with a P55 MB and the 750 CPU.