I just checked Tails 0.19 in VirtualBox. It uses 320 MB of RAM after TorBrowser has started. Then I started Pidgin, Claws Mail, GIMP, Scribus and Gobby and it was using 376 MB. Granted, I didn't do anything with these apps. I'm sure the RAM increases over time, so yeah to run many applications for a long time, you'll want more than 512 MB, but I think if you only use TorBrowser, 512 should be fine.