The question is, can you afford Nehalem? The board per se may come out to $300-400. The cpu is a question mark.
The sweet spot, IMO, is the P45. This time next year, you'll see what I'm talking about. People aren't Bill Gates and can't upgrade to Nehalem. The P45 will be around for a LONG while. Intel won't like it. Well, they made the mistake of forcing overclockers to buy the premium Nehalem boards.
If you're going SLI, look at the 750i/780i with DDR2 & Yorkfield support. They both do true X16 SLI. The P5N-E doesn't. Only x8 SLI. Also, 5x0/6x0i can't do 45-nm quads. So you can't throw the Q9450 in the 5x0/6x0i boards.
These are good w/c stores. I suggest you go with a pre-built kit such as the Swiftech H20-220 Compact CPU Liquid Cooling Kit. I don't recommend cooling the GPU's unless you can afford it. The GPU block for the new cards ain't even out. I'd say it'll cost about $100 each. You just have to cool the cpu so you can overclock it further than on air.