Could the 3.2GHz computer be dead, i.e. DOA? Also, Aero has transparency, which requires more processing power and more memory. And personally, it seems that Tiger doesn't have that many impressive effects. It might have a few hopping icons or pretty buttons, but thats about it.
It ran perfectly under XP for about 2 months prior to Vista's release.
Tiger has plenty of impressive effects under the Quartz Compositor and
Core Image engines, including Transparency, On-the-fly 3D, Real-time
Rendering, 3D Effects and Exposé. Microsoft has basically called these
DirectX 10 and Windows Flip 3D.
It's currently on par with Windows Vista in terms of special effects. But it
looks like the 2007 Mac OS X Leopard build has a new rendering engine
that works in place with Quartz Compositor and Core Image, called Core
Animation. This means the entire OS can be live-animated and rendered
in real time.
Of course this will need a bigger, heavier graphics card, but 64mb VRAM
should do. The baseline for the new rendering engine will be 32mb VRAM.