It seems KDE needs more than 512MB RAM assigned in Virtual Box, I'd put 800 something because I just have 2 GB and I was able to install and use KDE Mini and Full, if you have enough memory 1GB should be just right.
That was a good reminder about why this family choose to use LXDE for work related things, it is running with 512 MB but you can go lower if you need it.
KDE is only in my home desktop.