Well, 2 things I want to mention. 1) overclocking DOES lower the lifespam of the components you run beyond spec. How much is anyone's guess. and your mileage will vary. It's all a crapshoot for getting a stable overclock regardless of brand reputation. There are some things you can do to ensure a good overclock, but stability may still be a hinderance. and 2) you don't necessarily need SLI for a gaming PC. The new ATI x1900xtx and Nvidia Geforce 7900GTX probably will be enough muscle for any game you want to play even at higher resolutions. That being said you CAN go into SLI territory if you feel you want to/can afford it.
Now The problem is this. AMD is changing from the socket 939 to the AM2 architecture soon and Intel is moving to new CPUs soon as well. What does this mean? Well, prices on the socket939 boards from AMD may go down in price some. The downside is that as technology ages and becomes more difficult to find, you may have a tough time finding replacement components or upgrades. Not in the near future, I'm talking say 3-5 years depending how often you upgrade. You can either get a board using the proven technology or buy a new board that may be faster than the one currently available. The new boards may not overclock as high if you plan to do that, and they will probably be more expensive for the time being. The choice is yours though.
I will say this, $2k will get you one hell of a system.
Do you want to go with AMD or Intel and we can start there.