... if the graphic card has to be only DirectX9 compatible(older 8.1 cards with 1.4PS) or if it has to fully support DX9(PS2 or 3), and how much WinFlip utilizes feats such as pixel shaders.
You've addressed this question to Koji, so Koji will add.
As much as I understand, a DirectX 8 video card would be fine if it supported the hardware acceleration feature. Since DirectX 8 video chips do not have the feature built-in, they won't work for WinFlip.
======
To check HAL availability, one can run dxdiag > Screen tab. (See if it has Direct3D support)
======
A DirectX 9 compatible video card sure has hardware acceleration and Pixel Shader model, which is enough for WinFlip combined with True Transparency.
As for Intel GMAs, I know GMAs 900 and higher do have HAL, SM 2, and other features typical to modern video cards. Newer releases are even better (e.g GMA 4500), and not only for WinFlip.