Just like with Windows, it's really only going to make a difference if you were paging alot previously.
I know I could use 4GB - sometimes Logic Express will use 1.25GB or more of memory on its own if I'm using alot of tracks and also trying to use the Jam Pack software synths. Then again, I'd need a Mac that can see 4GB (Leopard doesn't help the 945GM chipset's problem with seeing only 3GB).
So the short answer is I don't think it'll be as big of a deal in Leopard as it is in Vista, and it depends on what you're doing, but if you're seeing paging issues now, doubling the RAM should drop that considerably.
I do not understand what I do. For what I want to do I do not do, but what I hate I do.