A friend played Warhammer Online with Crimson and I over the weekend on a computer that only just met the minimum system requirements.
Single core CPU 2.5GHz, 1 GB RAM, 8500 GT graphics card, running on XP.
The settings needed to be turned down ASAP to avoid lots of crash-to-desktops. Simply choosing the "best framerate" isn't enough, you need to go into the settings manually and decrease the resolution to minimum, remove fancy effects, etc. Removing names of enemies and non-partied allies seemed to help quite a bit in large battles/towns.
In the end, the thing ran pretty well, with hardly any crashes. Not bad for a $50 graphics card.
So for all those thinking about running the game with a similar system or better, go for it. If your specs are much less than that, you'll probably need to upgrade. Fortunately the 2 things most likely to improve your performance (RAM and GPU) are available very cheap.