Hello,
All you need is 512 MB, as you have it you should be able to go to that version.
Regarding the ASDM version, you will need at least to have the ASDM version 6.3(2), the recommended version for the ASA version you will be using its 6.4(7).
Do rate all the helpful posts
Julio
Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC