*standard OJN game recommendation, coming riiiight up*
Homeworld.
The single best RTS (and, IMO, game) of all time. 11 years old, but the only thing that's aged is the graphics.
A whole slew of my fanboyish reasons for choosing the pure awesomness of Homeworld can be found here:
http://www.escapistmagazine.com/forums/read/9.233628-A-Complete-Game?page=2#8250565
Other than Homeworld, the Mass Effect games are pretty good on the 360, I can't imagine they'd get any worse on PC. Same for Halo 2 (and the guy who said you should buy Call of Duty instead - not gonna name any names, you know who you are - well... catch fire and die, please). Dunno about Halo: CE, I haven't had the fortune of playing it yet.