5.0 (90) might be a little unrealistic. However, under 6.7 (120) is doable with some work. Of course, if one is on insulin, that changes things and (correct me if I'm wrong) would make it easier to get the lower numbers than if only diet controlled.
That said, I DO usually stay under 100 (5.6) most of the time, but I eat very low carb/high fat to do it. I will say, though, that this has been a very good way to keep my BG numbers steady on the low side.
According to my doctor, anything over 7.0 is diabetic and therefore likely to cause damage. I try to keep my 2 hour test at around 5.5 (can't seem to get it much lower). Breakfast is my biggest challenge; it's usually around 5.8, in spite of the fact that I'm usually starting at around 4.6-4.9 and consume few or no carbs at all in the meal.
140 (7.7) is a little high to be your main goal - it may be "okay", but it isn't particularly good. Some of us aspire to never go OVER that level, but to stay way under it most of the time and that helps in getting better A1c numbers too. The lower, the better. Stability is an issue also - try to keep wild swings to a minimum.