It's probably not so much a shader 3 issue. Many Intel graphic solutions also support shader 3. But not everything can be "fixed", just like that.
I have searched for, and not found.
A game using .NET/XNA + Direct3D + Pixel/Vertex Shader 3. Also with a lot of stuff moving around on screen. And a lot of light effects.
Only one game include all this. That's Magicka. Have never found any other game to compare with. (Done the same way, including same features.)
As of right now, even crappy Intel 4500 GMA's can launch and actually run this game. (Like crap, but still, Intel 4500 is like less then 10% of a GeForce 8800) If you use XNA in combination with shader 3, and those Intel GMA's can run the game. Already there is an indication that you in fact did a fairly good job regarding optimising. As long as all features remain, and the game is based on XNA. I doubt requirements will lower that much, if any. Based on what I read, I also doubt there's that much optimising that can be done. Optimising that also will, in any significant way, affect the game performance that is.
Also, one must realise. As long a Magicka runs well on a computer that meets the required specs. Devs are not obliged to do anything at all. The may try to, and I believe they have done. But are in no way forced to consider ppl. not unaderstanding their computer specs.
The "low requirements magicka" will probably also be a "2'nd generation magicka". And that might be quite a while before that happens.
Look for 60$ games produced with a XZillion bucks budget if you really need low requirements for the game. Many (most?) games from small/new/indy companies will tend to need a powerful computer to run.