You really should consider just building it yourself. Modern PC parts are basically Legos, and it's impossible to screw something up unless you can't follow some very clear directions.
At a $600-$700 budget, you can easily build something that'll run C&C3 and RA3 at max settings at a pretty high resolution. I'd recommend a tri- or quad-core Athlon II, Micro-ATX AM3 mobo, Antec Mini P180 (there's really no reason not to get this case no matter what your budget), and 4 GB RAM as a starting point. For the video card, what's your monitor resolution? If it's anything less than 1920x1080 or so, you can probably get away with using a Radeon 4850 or something similarly cheap.
"'Casual games' are something the 'Game Industry' invented to explain away the Wii success instead of actually listening or looking at what Nintendo did. There is no 'casual strategy' from Nintendo. 'Accessible strategy', yes, but ‘casual gamers’ is just the 'Game Industry''s polite way of saying what they feel: 'retarded gamers'."
-Sean Malstrom
