the mainstream nehalem at 2.66 Ghz is rumored to be priced at 284 per thousand, which means it is going to cost you about $350(after prices stabilize). the cheaper X58 motherboards are going to be about $250 and 3x1gb ddr3 ram is about $100. so you are look at about $700 for a entry nehalem system. right now you could buy a Q9550, p45 mobo, 4gb ddr2 ram for about 150 less. the nehalem system will have the better CPU and has less ram, but will cost more and you will probably be waiting until after christmas for things to be in stock. it took a couple months for penryn processors to be in stock all the time and i suspect that nehalem is going to be even worse.
I would get the proven winner today, because next best thing is always right around the corner, and if you keep playing the waiting game, you'll never upgrade and enjoy your new computer.