If I buy a 13" new Air, I am thinking of just getting the 128gb memory but upgrading to 8gb Ram, I have a 1TB external hard drive I can put most of my stuff on, or would you go the extra and get the 256 GB memory?
In order of importance for the Air:
1) RAM upgrade
2) SSD upgrade
3) processor upgrade
If you already have a desktop at home and this will be your secondary machine, 128GB is enough (assuming it's just for light use and traveling). If you plan to do anything more with it, 256GB would be ideal.