My system is a five year old P4 (1.7 Ghz, 512 Mb ram, 64 Mb video card, 70 Gb hard drive) and runs Civ 4. The wonder movies don’t really play but otherwise it works. (I did turn off unit animations because it was a bit too sluggish with them on, but that was it.)
From what I can tell Civ 4 is very tempremental as to which systems it runs on and which ones it won’t though.
Master of Orion II can throw errors at times in XP and can have other compatibility problems as well. I’ve noticed it most going into movies. Great game, though.
I upgraded my 5-year-old video card (which was great for its time) just so I could play Pirates and Civ IV. Hopefully my new card will last me until at least 2010.