First, don't buy your RAM through Apple. Everyone will tell you this. I bought mine through OWC (macsales.com). In the past I've bought it through Tekserve in New York. As long as it's the right type, you'll save a lot of money, and RAM is really easy to install by yourself.
I had 2 2GB modules and 4 512 modules, so I bought 6 additional 2GB modules for about $435. This gives me a total of 16GB RAM. If I wanted to get to the max of 32 GB, it would have cost me around $1,800. If you can afford the 32GB of RAM, I would do it. The sky will be the limit someday, when Vienna Ensemble becomes 64 bit (which I believe is scheduled for October). Plus Snow Leopard is about a year away, which promises to access a lot of RAM (what Snow Leopard means for us music types I have no idea -- I think no one does). Logic EXS is kindof 64bit already in that you can load over the limit (irrelevant if you're using Nuendo).
At some point, you'll probably hit CPU limits before RAM limits, and you'll hear the pops and clicks. With my new setup I'm taking some load off my G5 and putting more on the MacPro. We'll see how it works!