Well, since no one's answered my questions, I suppose that I'll have to answer them myself.
I decided to take a chance and buy the DPR-1260 from Microcenter. They have a 30 day return policy. I thought that if I couldn't get the printers to work after a bit of fiddling, I'd return it.
Samsung ML-1450
Where to begin. Short answer is that I can print to the Samsung from my three XP computers and my one Vista computer. I haven't had time to try from the iMac.
Installation went smoothly on the first computer. The ML-1450 was directly connected to this computer before I started all this. I disconnected the parallel print cable to be sure that it was printing through the DPR-1260.
Installation on the second computer was a bit more difficult. I made the mistake (as at least one other person did) of selecting the wrong driver on the first pass. I mistakenly selected a PostScript driver which is incorrect. Like the other poster, I could never find a way to tell the setup executable to look for or load another driver. Can someone post an answer to that question?
Anyway, with the wrong driver installed, I could connect to the printer via the DPR-1260. It's just that the output was gibberish. So, at least I had part of the problem solved.
I found this posting: http://www.dlink.com/support/faq/default.aspx?prod_id=2705&question=DPR-1260%20,%20DP-300U%20,%20DP-301P+%20,%20DP-301U%20,%20DP-311U%20,%20DP-G310
It shows how to manually install a printer in Vista and Windows 7. The instructions were close enough for XP. Installing this way allowed me to select whichever driver I wanted. After installation, I was able to print the Printer Test Page correctly through the DPR-1260.
Then, from the Printers and Faxes window, I selected my new printer, hit right click and selected properties. From the Port tab, I selected the port that the DPR-1260 had created when the wrong driver was installed. I saved that and was able to print to the printer using the correct driver and through the port configured by the DPR-1260 installation process. Then, after deleting any pending documents, I deleted the printer with the wrong driver from the Printers and Faxes window. (You can keep using the initial port, but it's an IP address. You have to have set the DPR-1260's IP to static. Otherwise, if the IP address of the DPR-1260 changes, you won't be able to print any more.)
The third printer installation on the XP went smoothly. The Vista was equally easy.
Epson Stylus Photo R380
I have only installed this printer on the three XP computers so far. This time, I was much more careful about selecting the proper driver. The installation went smoothly on all three computers. I actually don't use this printer much from these computers. (Ink is expensive. I print photos from the iMac.
The real test for me will be trying to install the printers on the iMac. I'm saving that for another day.
Ray