More details on the RAM:
The i3s, i5s, and i7s below the 900-series use dual-channel memory. It's best to get an even number of matching DIMMs: 2, 4, 6, or 8 sticks of memory.
The i7 920 and up on X58 use triple channel memory, or multiples of 3 matched sticks of RAM. You probably saw something about the 900-series that suggested 9GB.