Let Crucial tell you what memory you require; click here
Not sure about good ATI cards, I've always bought Nvidia; any card from the Geforce 6xxx range will do - Pixelshader 3.0 on those. I'm sure someone could suggest the equivalent ATI alternative.
You may find this a useful read too; click here
and this; click here
You don't need a spectacularly powerful card for that game by the sys requirements.