I had encountered the same issue, but that was pretty easy to get rid of. The screen of your Guest OS simply wont appear unless you click on that black screen. That's all.

However, if you really want to deal with increasing the memory, you mean primary memory and not secondary. Nonetheless, Here is a way to increase both of them.

**Primary Memory**

In VirtualBox, This is referred to as Base Memory. Right click on the desired vm, go to 

     Settings -> System -> Base Memory

And then adjust the memory as you need.(Just remember that your host OS also needs to breathe RAM)

**Secondary Memory**

Secondary Memory is where your programs are stored even if the machine is turned off. I prefer doing dynamic allocation when installing the machine. And in case one forgets to do that, this link has some [answers][1].

Hope that helps.

  [1]: http://askubuntu.com/q/88647