Just had a "small" battle consisting of 24.000 men. These armies where properly balanced so I'll try to make a battle consisting of only 500 men militia units in 6 armies total. My specs:
E6600 @ 3.0 Ghz
4096MB RAM
ATI XFX 4870 1gb XXX @ 4890 speeds
I think the minimum fps in this battle was around 15-20 but mostly it stayed at 35-40 fps!
edit: My Brits are frenchies too now!