Thanks for your messages guys- I am in touch with ATI technical dept prior to getting your recommendations also.
I have not done any overclocking as I was worried about overheating (and dont really know how to!).
I have read the links sent to me above also within this forum, and it would certainly appear that my card is an old one for this game requirement (especially with the many great new mods that I am running with the game), so will upgrade it asap.
Cheers for now