I thought it might be an idea to do some soak testing for a day or so. Are there any good tools for doing this?
badblocks

I was planning on creating a partition on the new disk the same size as my existing partition (I don't bother with multiple partitions) and then dding the whole partition across. Can I then later resize the 8GB ext3 partition, or would I be better off creating a new 30+GB data partition with the remaing space?
I think dd'ing the partition will only work if your new drive has similar geometry to the old one (i.e. same heads and sectors). IMO you're better off partitioning it the way you want and moving your files with tar instead of dd.

Peter