Finally got it working
I am replying to my own post because I finally got my Viewsonic VX2025wm monitor working on my PowerMac G5 with Radeon 9800XT. It is now hooked up via the DVI connector and running at 1680x1050. I spent many hours on this and I hope that sharing my experience will help someone else find the answer to their problem.
It basically came down to two things:
1. reset the PRAM (restart, hold down CMD-OPT-P-R, wait for the second set of restart chimes, let go)
This step may not be necessary, but it is what I tried first and it was what allowed my Mac to recognize the VX2025 by name and make the 1680x1050 resolution available as a choice in the Display preference pane.
2. update ATI video drivers. Download a file called macosx-ati-displays-4-5-7.dmg and run the installer it contains.
https://support.ati.com/ics/support/default.asp?deptID=894&task=knowledge&folderID=299
I installed, rebooted and the VGA connection was still working. (I wanted to make sure I hadn't made things worse.)
Then I plugged in the DVI cable and hit the "2" button on the front of the monitor. There it was. I finally had a connection via DVI!
In case you were wondering, I did consider the driver update early on, but oddly, none of my Google searches turned up anything indicating that such a thing was available and I couldn't initially find it on the ATI site. I am hoping that I used enough keywords in this post that it will come up if someone searches like I did.
On a side note:
Since I had swapped cables a million times while trying to get this all working, I decided to swap again to do a little comparison between the digital and analog pictures. To be honest, I couldn't really tell the difference. Of course, this was not an exhaustive test. I simply looked at the same photo with both signals.
I did learn that the Mac doesn't like to provide a digital signal everytime I swap the cables. Sometimes I could get it to come back by hitting the input selector on the monitor, but not always. I don't know what triggers the signal, but I ended up rebooting to get it to work. I don't plan on swapping the cables anymore.
Now I am off to enjoy the new monitor...