After a hardware failure of my NVidia 8800 GTS 512, I've been forced to temporarily downgrade to an ATI/AMD Radeon X1950 Pro - and Virtualbox doesn't like it one bit. Aero's a complete disaster on this card.
Host system specs: Windows 7 Ultimate X64 on AMD Athlon II X4 935, 4GB RAM, ATI/AMD Radeon X1950 Pro graphics.
Guest system specs: Windows 7 Ultimate X64, 2GB RAM. Virtualbox 4.1.2 / Virtualbox additions 4.1.3r73784.
I've attached a screenshot so you can see the effect.