Hi Mrs_CatholicPortugalian and welcome,
May I suggest that you turn down your graphic settings. You can do this going into options in the main menu, then into video options and clicking on the advanced button. Simply drag the unit and vegetation sliders further left and turn off shadows and other such features in the check-boxes below the sliders. Then you can scale down the unit size to something that your card would find easier to read. I always use the "normal" setting for my card, which is a 64MB GeForce 4 MX 420, and I only have small lag problems in large battles. Your higher spec card, being much better than my old piece of scrap metal, may be able to cope with the large setting, however, if you still experience the lag, then turning it down would probably be the most viable option. All you have to do then is start a new game and all the settings will come into effect. Sorry, but the unit size only changes with the start of a new campaign. My apologies. Hope this resolves your lag, cheers!