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I've had 512MB, 1GB and 2GB cards on my N80 and I haven't noticed any real change in performance.
A bigger card could theoretically slow things down if the card you bought had really high seek times and/or low a throughput, but usually the only real difference to performance will be where the entire card is being scanned - for example when you're updating the music player library. Obviously, if you're scanning 2GB instead of 512MB, it's going to take a little longer.