Well, everybody is different, so I don't know that there is any particular amount that's "perfect". There are some rules of thumb based upon your weight, but they seem rather arbitrary to me.
My advice is simply to drink as much as you are comfortably able to. Personally, I drink at least 3 liters during the work day (7 to 3 in my case), and at least another liter at home (for a total of about 1 US gallon). Really, within reason, the more you drink, the better. I prefer ice water to help burn more calories, but that's up to you.
47 M 5'8"
May 4, 2010...... 440? lbs. (Start FitDay Size 60 Jeans)
June 19, 2010.... 393 lbs. (First Weigh-in)
June 19, 2011.... 229 lbs. (164 lbs. gone in one year :-)
Current Weight... 185 lbs. (Size 36 Jeans)
Next Goal 169 lbs. (07/04/12)
Ultimate Goal 165 lbs. (12/31/12)
The best exercises for weight loss are Fork Putdowns and Table Pushaways.