I'm surprised nobody has mentioned VGAP 4. Sure, it's getting a bit long in the tooth, but it is (to me) the space 4X equivalent of Dom3;
- complex
- significant race differentiation
- turn based
- end game is equally time consuming
- same high quality graphics and audio
- SP is worse than Dom3, but MP is better (allies are recognized)
It's relatively bug-free (although I haven't tried it with Windows 7). Unfortunately it is an indie product whose designer has limited time for further development. Anyhow, if interested, the best place to start is probably vgap.drewhead.org/