I play with 1gb of ram and a Go6800 and I don't have any problems. I did have to have a lot of the eye candy off, but it still looks better than rtw ever did. Turn grass to low or off and turn shadows to low. That bumped my fps up about 10 or so which made a huge difference. Also turn buildings to low. That will help in siege battles. Just turn everything on low and then slowly add more eye candy till it makes it jerky again and turn the settings down a notch. See how that works for ya. Do some tweaking before you dump any money into hardware.

Another gig of ram probably isn't going to help your situation any.