Note that RTW didn't exactly recommend this battle setup, though: it starts complaining at about 7K that this 'could impact performance'. I did have slight lag, the worst was rotating the camera because it forces re-computation not just of the entire screen, but also of (part of) the world surrounding it. However, ordering troops wasn't laggy at all (the biggest lag by far occurred between the keyboard and the brains) and neither was the AI.

Specs are a 4870HD, the 1GB DDR5 edition (it was only 10 or 15 euro's more expensive than the 500Mb version not such a bad deal) from Club3D who have had their way with it first (they replaced the stock cooler with an massive piece of aluminium and fan) coupled to an Intel E8600 with stock cooler and some 2500MB of RAM for RTW on its own. Perhaps I should try that application header setting mentioned elsewhere. Might help stability.