http://spring.clan-sy.com/ nuff said, try it. It has multiplayer online and bots you can play agains't as a beginner. It has awesome mods available at www.unknown-files.net (for some reasons the dled files are missing the dot between file extension and filename, eg mymapsd7 rename them to mymap.sd7 (temporary issue)) try it, it has a lot more playability than supreme commander and it runs smoothly, even on older machines, over 1 ghz is however recommended. And there even is a linux version
