Excel v.X runs fine under Leopard, I had been using it on my Macbook Pro Core 2 Duo 2.33GHz prior to and after upgrading to Leopard back in Oct.
Only just last week did I upgrade from Office v.X to Office 2008 (I never had Office 2004)
I don't have any quantitative measure of relative performance between v.X and 2008, but it feels basically the same, in other words I think you'll find v.X under Leopard to be fine.