Okay lets first start by saying that your Core2Duo will be a slight bottleneck for the 5770. That being said maybe if you can overclock it to over 3.0ghz it won't be a bottleneck. But to answer your questions.
Flight Simulator X (Medium-High).
Call of Duty 4 (Ultra High)
Modern Warfare 2 (Very High/Ultra High)
World of Wacraft (Maximum Settings).
This is with a 5770 at 1080p settings. 1920x1080 Resolution. Hope I helped!
Huge improvement. The HD 5770's performance is almost as good as the HD 4870 which is a card from "the same generation" as the GT 8500.
The GT 8500 is a budget that is considered more of a card for an HTPC (home theater PC) than for gaming. The HD 4870 was one of the best single GPU video card before AMD released the HD 5xxx series in the fall of 2009.