I have a fairly high spec machine:

P4C @ 3.7ghz
1 gig DDR500
6800GT @ 415/1100
2 x 120 gig Maxator hdd's (Raid 0)

One of the first things i did when i got the game was tried a HUGE battle. Had every setting on highest (except AA which was at medium). Setup a bridge battle with 5 armies all fighting against me (just for a laugh :D ). Must have been about 10000 troops - RTW warned me i had exceeded the recommended limit (etc etc)

When the game started it wasn't too bad, but as the enemy troops all started crossing the bridge and my 8 archer units let loose with fire arrows i started getting about 1 frame per 5 seconds :(