I'm not sure what to install on my windows box.

My sites portfolio:
4 php sites
2 php sites using mod_rewrite (currently using isapi_rewrite for windows)
2 asp .net sites

IIS would be able to run them no problem, but I'm wondering if perhaps using apache would be better on cpu/ram usage. aka use less resources.

Any advice?

You would not be able to run your 2 .NET sites on Apache.

perhaps using apache would be better on cpu/ram usage

ON a linux box.... yes. On a Windows server... NO! (not that you could run IIS on Linux but that's not the question here :) )

http://weblogs.asp.net/israelio/archive/2005/09/11/424852.aspx
says he was able to run it, hence my consideration of apache.

about your bold "NO!"; Does that mean apache would use more, or they pretty much equal.