I had a AMD X2 4200+ and a 7900 GS when I started playing Warhammer. Very poor performance with min settings@1440x900. I upgraded to a 9800gtx+ and saw some performance gains, though still unplayable in keep sieges, I'm talking sub 10fps in min settings@1440x900. If you OC your CPU you will get much more performance out of warhammer. I ended up OCing my 4200+ from 2.2ghz to 2.5ghz, though still the game was unplayable in keep sieges@min settings.
I upgraded to a i7 920 and, even at stock speeds, I can run at High Quality with frames from 40-99 fps on 1920x1080 (got a new monitor) with my 9800gtx+. In high # keep sieges, I'm talking 150+ on each side, my frames can drop to 30 fps. Still HQ and 1920x1080.
Believe me, you will see much better gains by upgrading your CPU! of course that means a lot of $$$... but you won't see much gains if you upgrade your video card alone.
Basically a new GPU will boost your frames at times when not much is going on, but your CPU will cripple you when lots of things are going on (you know what i mean).