Darwinia, has a mix of everything. Strategy, shoot em up and puzzler. Imagine a mix of Cannon Fodder, Lemmings and Populous and you're somewhere there. And its only a tenner if you download it from the developers website www.darwinia.co.uk. D/load the free demo first to give it a go. Its also by the same lot that made 'Uplink', which I bought from a recommendation on this very forum.
-tactics play a major role, you can have the best units are the germans, and still get beaten back by inferior allie armor, just because their tactics are better than yours!
-the graphics are superb, matching the quality of any modern FPS. I enjoy just setting up a massive battle, and then pausing to take screenshots!