The Canon EOS 5D Mark II offers breakthrough 21-megapixel resolution at a relatively low price of about $2699, or about $3499 with the 24-105/4L zoom (about $1049 separately). Those 21 megapixels make a substantial difference in detail over 12 megapixel cameras like the Nikon D3/D700, as documented in DAP. But to get everything possible out of the entire sensor, you’re going to need top-notch optics. But remember, a shot not taken is zero megapixels, so don’t get hung up on the “perfect” lens.
The Canon EF 24-105/4L is a handy all-around lens, and it’s a wonderful match for the 5D body in terms of size, weight and just shooting fun and convenience, so I recommend owning one (a good copy of course). But do not expect it to deliver top quality across the entire frame, even stopped down to f/8; central 2/3 sharpness is superb, but it’s downhill from there, at least my copy is. The best way to get one is to save $250 by buying it with the 5D Mark II. The 24-70/2.8L (about $1110 after rebate) is a better performer, but not nearly as fun to shoot (ergonomically awkward compared to the 24-105).
So what other lenses should you match to the 5D Mark II? Forget about f/4 for low-light shooting; the 24-105 will frustrate there. But like the 24-105, the 70-200/4L IS (about $1025) is a pleasant lens to handle and shoot with—those two f/4 lenses make a terrific lightweight and relatively compact travel duo for the 5DM2.
You should have at least one low-light lens, but f/1.8 or even f/2 is fine for that. For most users, f/1.2 is overkill, hard to focus accurately, heavy and expensive.
For low-light shooting at a low price it’s hard to beat Canon’s 50mm f/1.8 II (about $89). This is your throw-away “sand and salt” lens! Sharpness is excellent. Bokeh will be better with the 50mm f/1.4 USM (about $325), but won’t matter much in the f/2 - f/4 range. The Sigma 50/1.4 EG DX (about $499) is also a very nice choice for (DAP report coming).
The value-priced EF 85mm f/1.8 (about $355) is an excellent choice also. It’s not f/1.2, but it doesn’t focus shift either! So it’s darn sharp at f/2 in practice with no fussing around (extreme corners do drop off abruptly, but the rest of the frame is outstanding).
Of course, you can spend a lot of money on the fancy medium range “L” glass: the 24/1.4L II, the 35/1.4L, 50/1.2L, 85/1.2L and 135/2L. In order of most useful, I’d choose the 35/1.4L ($1180), 85/1.2L II ($1829), 50/1.2L ($1400) and 24/1.4L II ($TBD). But it all depends on what you shoot. These lenses are useful if you regularly need to shoot in the f/1.4 - f/4 range.