It plays fine on my bargain basement machine;

2.5 ghz athlon
~700 meg ram
Geforce 3

The graphics are minimum specs, but as the ram and cpu are good it runs fine. I cant get all the nice effects and it slows above ~3000 soldiers displayed (so no huge battles. ). It cost me ~200 quid to put this machine together (self built & cannibalised), but it runs RTW fine and looks pretty good so you dont need to spend the earth. depends if you want to play doom 3 too I guess.