I think that is more than powerful enough. 3 GHz Core 2 Duo/ 4Gb / 9800 GT - I did find that Warhammer runs fine with less RAM and with older video cards, but the savings in overall system price might not be significant with current hardware choices.

What I would consider is what else do you want this computer to play? You're right investing in hardware like that to play War hammer online is expensive. No matter what you buy you might come up short in game due to the game's mechanics as it relates to multibox play style. My advice: make sure it runs your other games/future games well enough as your experience into Warhammer might be short-lived, resulting in buyer's remorse.

And for what it's worth for me 5boxing Warhammer felt way way too much like a Chinese fire drill.