rockmedic's advice is pretty spot on. CPU horsepower is what you'll need. Core i3 is considered a good value, but if you can spring for it, get an i5. An i7 is thought to be overkill at this time for gaming since not many of them take advantage of the extra threads available on that CPU, and the price/performance ratio is generally not worth it. This may change in the future, but certainly not the case for WOFF, unless they decide to create their own engine (unlikely).
Besides the GPU advice, I'll add that you should look for something with at least 6GB RAM, 8GB would be better. You can run it on a 4GB system, but more data will have to be read from the disk on the fly which may translate into increased stuttering, slowdowns, etc.
Also I highly recommend going the SSD route. With the falling prices of these units, it doesn't add a significant premium to the overall price as it used to. You'll be glad of this when you see how much faster the missions load.
