If you're not too attached to Unity, try Lubuntu or Xubuntu. Both work rather well in a VirtualBox. The last Ubuntu that worked lightning fast in a VM was 11.04.
I moved on to Crunchbang. It's noticeably faster than even Lubuntu in a VM. I don't think even the older Ubuntu's were this fast. I dedicate 32M of video to Crunchbang and have had no problems.
I moved on to Crunchbang. It's noticeably faster than even Lubuntu in a VM. I don't think even the older Ubuntu's were this fast. I dedicate 32M of video to Crunchbang and have had no problems.