Since you're running a Linux guest in a Mac host, I'm sure little of this applies, but I thought I'd mention it anyway.
Display resolution was the first thing I fought with. Upon initial install of Windows XP as a guest, it gave me the 800x600 resolution, which I just can't deal with at all. Using 16MB of video memory, I was able to change that via the standard Windows method up to 1024x768, but I could go no further. Still a bit small for my tastes since I'm accustomed to 1600x1200. I then read some of the manual, and it suggested I needed to install the VirtualBoxGuestAdditions.
Well, that didn't seem to be anywhere.
I then discovered I had installed an older version of VirtualBox (1.56 via apt-get) that didn't have this. So, I uninstalled that and downloaded the 1.62 (latest) from
Sun's website. From there, I booted the virtual machine and found I suddenly had new resolutions available to me already (up to 1280x1024), and the guest additions ISO was available to mount as a virtual CD/DVD, and I installed that as well. It improved performance some.
The two things I got out of this initially were that a) the latest version of VirtualBox has more features and b) the guest additions stuff improves performance of various things similar to how a driver for certain kinds of hardware would.