Two RTS (real-time-strategy, ie. like C&C) games I'd recommend:

Warhammer 40k: Dawn of War
Lord of the Rings: Battle for Middle Earth

Both are worth buying, if you're into RTS games, to be honest......Dawn of War doesn't need an amazing computer to play it, but it's a little more forgiving than BfME is.

One word of warning, though, since you didn't actually specify if you were going to buy or "buy" them......BfME can't be played online, or on a LAN, unless it's a genuine version (your base will explode after 5 minutes, killing you, if it's not genuine). Quite amusingly, from memory, someone posted on the official forums asking why his base exploded part-way into a game, and one of the designers pointed out that only pirate-versions did that; the complainant didn't complain on there any more, surprisingly!