Well, I fitted the RAM and after a few snags, (I dislodged the video card without knowing and was scratching my head for a while!) it works fine, even though they are different speeds. I played Farcry and some other games and the extra memory makes a massive difference.
Anyway, yes I do have AA enabled but I did last time I benchmarked it, yet it was higher, which is what I don't understand. Might disable it anyway though.