MSI P7N SLI Platinum LGA 775 NVIDIA nForce 750i SLI ATX Intel Motherboard - Retail
ASUS P5N-D LGA 775 NVIDIA nForce 750i SLI ATX Intel Motherboard - Retail
EVGA 123-YW-E175-A1 LGA 775 NVIDIA nForce 750i FTW SLI ATX Intel Motherboard - Retail

I am building a new system and the only thing I am stuck with is the mother board and the memory. I was suggested with the above three boards. I just want a board that is going to provide me the fastest speeds and best performance.

I have several questions and I am hoping you guys will be able to help me:
1. Which of these boards do you suggest? (I don't care about the price)
2. One of these boards has it's memory standard set to DDR2 1066, while other two are DDR2 800. What does it mean? Does it mean that the one that has DDR2 1066 will be able to work with a faster ram?
3. Meanwhile can you suggest me with a nice ram? I want 4GBs of RAM and I have up to $170 to spend on it. I WOULD REALLY APPRECIATE if you gave me direct link to the product that you suggest (preferably on newegg.com)
4. What does PCI Express x1 mean? I want to buy two video cards and go SLI on it. Each of those boards above have two PCI Express 2.0 x16 slots, which mean I can plug 2 cards in. Do I need same amount PCI Express x1 slots to go SLI?

Thanks in advance....

Here is a set up of my future system, which does not include the mother board and ram yet: