I have an old Pentium III 700, and I have no problem with MTW:VI, other than loading up the freaking eras when starting a new campaign or custom game.
My video card is exactly the same as yours (G4 Ti4200), but I have more memory (384mb), so that may be it. If you can, nab yourself another 128mb memory chip if your computer can handle it. Now in the days of memory going up in the gigahertz, you can easily grab yourself a good memory chip for very cheap at newegg.com or other distribution sites.