Your CPU is going tio grind, abd your GPU is a joke.
Leopards fancy shader code for menus, copied from Vista for no apparent reason, and reflective dock will kill your graphics performance.
In iChat you won't have access to any of the iChat theatre effects, your CPU and GPU won't be fast enough.
You won't be able to use de-interlacing on the DVD Player as it requires a 1.6Ghz processor or faster.
Spaces will cause a lot of graphical tearing.
Basically, Leopard is a monster upgrade and it requires a newish GPU and a dual-core processor to really shine. MBP, iMacs and MacPros run this good.
No doubt, someone will reply and tell you that they are running Leopard on a Mini and all the effects switched on. When they type that, their monitor will explode as their nose is going to be growing faster than pinnochios.