I find that for typing, having the keyboard on my lap (towards my knees) to be the most comfortable spot. My arms are nearly 90 degrees, if I need to I can rest my forearms on my legs, and can sit with good posture.
As for the desk, I prefer to have the keyboard a little further away so that my wrists remain more straight. It also helps to make sure your chair is high enough, and that your arm rests are not too far apart. Unfortunately, most chairs leave lots of room between the arm rests, but some chairs let you move or angle the arm rests inwards, which can help dramatically.
My current chair, while very nice, is a little too low and the arm rests are only adjustable vertically, so I just put the keyboard on my lap when I do much of my typing. It also makes the keyboard a bit quieter!
Also, when gaming (which involves using exclusively my left hand), I will position the keyboard at an angle that keeps my wrist straight, and I will vary its position on the desk.