Starcraft 2 if you're more into the RTS part of RTS and less in the building part.
You're crazy. I played SC2 on low settings on 60 fps with a freaking ATI Mobility Radeon 3270 with 128MB VRAM. SC2 is incredibly well optimized. It can run on almost anything. I now play it on ultra with 60 fps on a 150 euro graphics card.I have a 1 year old computer and on the lowest graphics its pretty chuggy, people told me to get a new graphics card, but I'm not that willing to spend 200 EURO for a single game.