More than 2 cores in general. RTW consists of exactly 2 processes: (a) RTW.exe; (b) e~005 or something which appears to be a kind of runtime or garbage collector.

So it is better for RTW to have 2 cores at higher clock speeds than 4 cores at lower speeds because it is not going to put more than 2 cores to any good use. So a high end Core2Duo is going to at least match if not outperform any Quad Core on this game (especially since the cheap Quad Cores sacrifice on the cache size which means that you are going to waste more clock cycles fetching memory from slower memory, namely RAM).

And some Quad Cores are more Quad Core than others.