The 1.8Ghz is just Core Duo, whereas the 2.4 is Core 2 Duo, so you'll be able to get 64-bit functionality out of Leopard.
Also, the 1.8Ghz uses an X1600 as it's graphic chip whereas the 2.4 uses an 8600M, which thoroughly whomps the X1600.
There are also some smaller differences (LED screens, etc), but those are the two main ones that come to mind.