What is "best" is very subjective and greatly depends on what you want to do with this machine. Do you want to Crossfire/SLI? Do you want to overclock? WIll you be using an aftermarket cooling solution? Will you put you drives in RAID? And so on and so on...
For an Intel based machine, an i7 920 is the processor of choice with a Core2Quad Q9550 as the next best and then to a Core2Duo E8600; in my opinion. The i7 is Skt1336 and uses the X58 chipset and the Q9550/E8600 are SKt775 and use any number of chipsets, most notably the X48 and P45.
For an X58 based board, basically any mobo with an X58 chipset will be a good performer with the primary differences being features and layout. X58 mobos can cost anywhere from $150 to $500, depending on the manufacturer and value-added features. THG has a great article on sub-$200 X58 mobos of which any would make a great foundation for a gaming machine. Note that just because a mobo costs more does not necessarily make it better, base buying a mobo on what you are going to use it for, the features you want/need, and the amount of money you are willing to spend.
If you opt for Skt775, then I recommend a high end quad core (9550) and a P45 mobo. Again, purchase the mobo based on how you will use it, the features, and your budget.