I can't really comment on Red Alert 3 as I've yet to play it, but C&C 3 is definitely very nice and worth playing. Even as a casual RTS player you should enjoy it. Nice environments, good graphics, and loads to see and do in the game. It also reverts back to a game style closer to the original C&C games, instead of the closer in view and very different style and game world used in C&C Generals games.

However I would warn you against getting the Kane's Wrath expansion pack (unless you can get both the original C&C3 and Kane's Wrath in a bundled pack. The reason I don't recommend Kane's Wrath is because it is aimed at the dedicated fans of the series and turns the difficulty level right up, so if you are only a casual RTS fan you might quickly become frustrated by the games difficulty level.

I definitely think Starcraft 2 is going to be something special. I'm definitely looking forward to that one, as are a large majority of any sane PC gamers.

There are however loads of other RTS games on the PC released over the past year that you might also enjoy and also be able to buy at cheap prices now. The space RTS Sins of the Solar Empire is highly recommended by me if you like space opera style RTS games. And the latest expansion for Warhammer 40K: Dawn of War is just out and well worth playing.

Other recent RTS games you might like to look at include Empire Earth III, Act of War, Maelstrom, Paraworld, Rise and Fall - Civilization at War, Rise of Nations 2 - Rise of Legends, Spellforce 2 - Shadow of Wars, Star Trek Armada 2 (you mentioned the first one, second one is better), Star Wars - Empire at War, Supreme Commander (one of the best RTS games of all time), The Settlers VI - Rise of an Empire, Universe at War - Earth Assault, World in Conflict, Warcraft III plus expansions...

Quite a few great RTS games. All of which you should enjoy, and many of which have now been out for some time, so you should be able to find them cheaply now.