Some pros and cons for the games mentioned above (ive been playing these for yrs)
Starcraft pros
-Great replayability (always new maps on b.net or make your own)
-3 totally distinct races
-hotkeys
cons
-alot of ppl hack...

... but i still manage to win most of the time, been playing for 7 yrs!
-steep learning curve if ur not good at strategy games, but if your good at WI, you should have no problem
C&C pros (not generals, the older 2d iso view, red alert, TS, red alert 2)
-Basically WI with super weapons and more variety(gdi and nod have some distinct units)
-random map genrator
cons
-after a while, game starts slowing down (i got a p2 400), when u got like 50 tanks (does same thing with WI on my tungsten)
the Age series is pretty fun too, from microsoft
Homeworld is a great rts, 3d space, might be a little tuff to control for newbs. but ive been playing games since 4th grade (we had a comp class which all we did was play adventures games

)