Well I thought I would never be able to run RTW at first but when I tried the demo it ran very smooth and the retail version ran even better.
I use "huge" setting and only have lag on really huge battles.
I don't think the specs for MTW2 will be very different from RTW, maybe a little increase like 1.2ghz processor instead of RTW's 1.0 Ghz.
CA are a pretty good at optimising their engines as well.