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> What governs the max. available resolution for the VM?
I'm not overly familiar with vbox internals, at least when compared to vmware, but my own educated stab in the dark would be differences in the guest-level X11 version and/or graphics drivers.
Modern hypervisors essentially allow guests to (appear to) "talk" directly to the host GPU / video card / driver, while my understanding is that vbox doesn't support this technology and doesn't plan to in the near future. Instead it still uses "virtual graphics cards" which emulate, to the best of their ability, the guest's configured video setup in software before passing it back to the host for rendering.
If guests are indeed responsible for their own graphics compositing, and given that different Linux distributions and versions thereof can come with wildly different kernel (and therefore driver) versions as well as opensource or proprietary videocard drivers etc, it would follow that different versions and distributions would have varying degrees of graphics performance when virtualized in this way.
You can probably fix just about anything by dicking around with your guests' Xorg configurations (http://linux.die.net/man/5/xorg.conf), however you might find that the cure (or at least the path to it) is worse than the disease.