I have 1.6ghz duron, 512ddr 333mhz ram, 128mb radeon 9600xt and I ran a battle v 7 computers, maximum unit size, maximum money, and I had biggest castle and 16 units of 200, like the computers.
And it didn't work anywhere near tolerably. It was horribly choppy, like reading donald duck cartoon or something...
And it took like 5+ hours before I won the battle, with minimal casualties.

I wonder if RTW will run well on me with big armies.. I've upgraded to 768 but it's still not quite enough I think..