I too noticed that the phone slowed down with a 1 gig minisd, which is why i had to downgrade to the 512. However, I find that the 512 is more than enough for my needs as I don't watch films on my phone like some people do. I just need it for the pictures, mp3's, and video taking.
Furthermore, I think the N70 is one of the best phones out there, and I wouldn't trade it for anything in the world....well except for a Ferrari Enzo