1. No, don't set it to 100%, you can shortened the card lifespan, let it set to AUTO.
2. Shader Clock only "visible" for NVIDIA card, ATI has different name but with the same purpose, don't worry about that.
3. HD4650 is 2.0 card, it's backward compatible with older board which has 1.0 or 1.1 pcie slot. So as long as your card is 2.0 you'll be fine (don't get 2.1 version).
4. Graphic card can stand above 100'C, but normally people would not get it to above 90'C when playing (full load) games. So 76'C-81'C is normal at full load.