If you want a Nikon that closely matches Canon’s 5D Mark II, you should take a look at Nikon’s D700 ($2,700).
Nikon’s pro-level shooter has a full-frame 12.1-megapixel sensor that performs very well at high ISOs, thanks to the large photo sites and Nikon’s advanced noise reduction technology. Also, the 51-point autofocus is nice and fast (faster than Canon’s).
If you’re in the market for something cheaper, Nikon’s D90, a 12.3-megapixel DSLR, is an excellent choice at $900 without any lenses. Or, if money is no object, step up to the D3X, Nikon’s 24.5-megapixel beast. It only costs $8,000.