I have the same Dual G-5 tower with 4GB of RAM. I also have a dual Intel Macbook. Both are running 10.4.x.
Some thoughts:
1. Epson is supposed to release a fully-featured driver for my 3800 and Leopard some time in March. We'll see. There is a
driver out there now, but Epson says it has limited functionality (and I haven't cared to find those limits, thanks.)
2. Photoshop CS3 is optimized for the Intel chips. I have installed it on both my tower and my laptop. In the Intel laptop, I see a good speed increase over CS2 (which had to run under Rosetta). In the G-5 desktop, it's actually a little slower -- CS2 runs faster on that machine.
3. Our graphic designer has a new dual-quad core Mac Pro with a ton of RAM. That machine just screams. I still want to test it with some raw processing actions, but my first impression is that there is a tremendous speed increase over both my tower and my laptop.
4. Everyone else in my office is running 10.5 happily. (But they don't have odd peripherals like the Epson printers.)
5. As much as I would love a new Mac Pro, my old G-5 tower is plenty fast enough for me right now. That might change if we can find some money to upgrade cameras, but probably not.