System Requirements will be higher than that of WC3 TFT. This is because of the large amount of Jass code running in the background. Also the textures are much higher quality then the ones in TFT and more units will be on screen so a better GPU will be required. Most of the staff have mid end to high end computers as we use 3D Max and Photoshop to create the art. The following is an estimate based on what some of the staff uses.
CPU:
Any AMD X2, AMD 64, Intel P4 (at least 1.8 Ghz), Intel P-D, and Intel Core Duo (2)
GPU:
Any mid end card from the ATI x800 Gen or NVIDIA 6600 Gen will be fine. I am not sure if WC3 can use Crossfire or SLI so there may be no benefit given from them.
RAM:
I would recommend at least 512 MB if you are running XP which is standard for any new games.