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#21
The current P67 board have 4 DIMM slots. They support 4GB DDR3 DIMMs, so that's 16GB. Some manufacturers have stated that they'll support 8GB DIMMs, when they become available.
Some of your other questions:
X58 is for Socket 1366. P67 is Socket 1155 (Sandy Bridge.) Socket 1366 CPUs support triple channel RAM, while Sandy bridge is dual. (However, the dual channel performance of SB is considerably better than that of the first generation dual-channel Socket 1156 Core I7/I5 CPUs. By one benchmark, it's 84% of triple channel for the same RAM frequency. From triple/dual, you'd expect 67%.)
Corsair is a good memory brand. Other makes may be a better deal, however. (Example: Gskill. Buy carefully, though: older series can be priced higher than newer ones with specs that are as good or better. I have no personal experience with Gskill, though.)
SSD? You might want to try to find one with a Sandforce controller.
Let me repeat: the Sandy Bridge CPUs use Socket 1155. The I7-9XX chips are all Socket 1366. They require different motherboards.
One other thing: if you want to overclock a SB processor, be sure to but one of the models with a k suffix (I7-2600k, I5-2500k). They have unlocked multipliers. Unlike the first-gen I7/I5 CPUs, they essentially cannot be overclocked by raising the bus frequency.