I've just created my new wonderful and delicious virtual machine (over VPC 2007) clicking on next, next, next :-D
I installed Win2008, MOSS, SQL 2005 and everything was running fast and fine while I tryed to install VS 2008 and realized that next, next, next wasn't a good idea (always think to what you're doing!). 16 GB weren't enough...
Instead of adding another virtual disk having 2 differents VHDs, I found this great tool:
It can resize a VHD and it works fine!
Obviously after that the VHD has been increased don't forget to extend also the NTFS volume (using the Disk Manager) to leverage the new vhd size!