C:\Program Files\Sun\xVM VirtualBox>VBoxManage createhd
VirtualBox Command Line Management Interface Version 3.0.0
(C) 2005-2009 Sun Microsystems, Inc.
All rights reserved.
VBoxManage createhd --filename <filename>
[--format VDI|VMDK|VHD] (default: VDI)
[--type normal|writethrough] (default: normal)
The command for creating images in virtualbox dosen't support that, tho its vmware varient seems to. Creating a vmware vmdk would be one option, but who wants to install another installer? There's better approaches
There's a few approaches you might want to try. For one, split up commonly used partitions from less commonly used ones, for example /home will be altered more often, as might /var. so having them as seperate disk images would help a littlr.
Switch to differencing disk images - this would create files with just the changes. Even more efficient bandwidthwise, and you'd only move the changes.