Uninstall Supreme Commander.
But I will try and compare the 2 for everyone ;)
If you haven't played the C&C games before, you will probably be wondering what all the gee up and hype is about this world. As a game, it has covered the 3 most important fun factors of an RTS game.
Simplicity.
Speed.
Destruction.
There are reasonable amounts of units and upgrades to experience in this awesome blast fest. Supreme Commander may be bigger and have more options to cover these areas but geez, doesn't that explain why it's so massively heavy on your pc?
C&C 3 has the perfect balance of total awesome explosive gameplay, and you can really pump up the graphics on a medium end pc NO PROBLEM.
This game looks beautifull. The lighting and shaders make your pants fill with many bodily fluids. The explosions are bigger and brighter than ever, and the cutscenes are classic! (Some great performance by the actors for once) and it is presented in Hi Definition - at least from what my monitor is telling me!
The balance of units amongst the 3 factions are awesome and very constructive. Each faction has it's own unique play, therefore you will feel the value for money aspect right there. The construction options are better than ever, as is the upgrading and All in a nutshell, the combined warfare elements, graphics and awesome programming of C&C 3 make it right up there in the RTS hall of fame. It beats C&C Generals and is faster than Supreme Commander. What more could you want???
Worth the bucks my friends, and it's not buggy at all. (at least not yet, I've just finished the 1st ACT)
Have fun!