It would be generally more accurate to say RAM = Good unless you're only doing basic word processing and internet surfing. GHz = Good if your app is a "number cruncher".
Although I'd think of it this way: the difference between 1.83GHz and 2.0GHz is about 9%, so at best that's the speed increase you'll see between the two. It's there, but it's not huge for most things, and generally won't even be noticible.
The difference between 512MB or RAM and 1GB on OSX, however, is likely to "feel" like more than a 9% increase. For any given operation it might not amount to that much, but the overall feel of speed will be greater, since OSX LOVES RAM. Now, 1GB vs 2GB may not make as much (if any) difference, depending on what you do--that boils down to the particular app you're using, how many apps you like to keep open, etc.
One more thing to keep in mind: The Core Duos in all current Intel Macs like RAM in pairs--that is, two sticks of the same size. They get about a 3-5% speed boost from pairs of RAM, although I've seen benchmarks in one particular game that showed a much larger gain for some reason. Generally the benefit is minimal, but if you have the choice between one 1GB stick and two 512MB sticks, you're going to get a slight speed boost by going with the pair. Of course, you'll have to chuck both if you later want to go to 2GB, so you'll have to decide on a priority.
