you can find power supply calculators if you search for them. eXtreme power supply calculator is a relatively good one. I recommend checking that out also when you get a chance. It's actually surprising how little wattage you actually need sometimes http://extreme.outervision.com/psucalculatorlite.jsp
5770 is 25 Watts lower power consumption than 9800GT.
3850 is 23 Watts lower power than 9800GT.
I don't know where you got those numbers from but they aren't right. The HD5770 uses about 10w less under load and the HD3870 5w. And that is only in comparison to the older 9800GT with the power connector. The newer version uses significantly less power than either of them.