Really depends if you want to play more demanding games now or in the near future, or if AoM and similar are going to be all you play.
If only AoM and other non-demanding games, any $100-$150 nVidia or ATI card will be fine. Look for the nVidia 5400 or Radeon 9200 or 9600.
If you want to play games like BF1942 and Fry Cry as well, I'd recommend going with a Radeon 9700, 9700 Pro, 9800, or 9800 Pro. If you can buy online (i.e., with a credit card), you can find them pretty cheap at www.pricewatch.com -- Just stay away from any with an SE in the name (i.e., ATI Radeon 9800 SE), those are crippled cards that are comperable in performance to, say, a nVidia 5400 or Radeon 9600.Hope this helps.