When connected with the VGA cable I get 1920X1080 resolution, but when I connect with the DVI-D/HDMI cable I can only get 1280X1024 - and the picture is stretched. How can I fix this? The Mac is a G4 (AGP graphics).
I doubt that the drivers for the old Rage 128 Pro AGP card will support that configuration properly, and I bet you don't have any luck with HDMI, probably because your vid card doesn't understand the HDMI connection at all.
What version of OS X are you using?
I know that OS X 10.5 doesn't have full drivers for that vid card. It's one of the reasons NOT to use 10.5, but stay with an older version of OS X. And, older versions of OS X would probably not have complete support for an HDMI/wide screen display. It's just too old.
You COULD try a different DVI to HDMI cable.
BUT, if you have the native resolution for your Dell display with VGA...
Why not just use that?
Is your vid card an original card for that PowerMac?
I don't think so, as the original card would have the Apple ADC connector (in addition to the VGA), which is a DVI connector that only Apple used.
The ADC connector is oval, not rectangular.
When you mentioned dual or single DVI, then you must have an aftermarket card.
If it was NOT manufactured as an Apple card, then you (or someone else) "flashed" a PC graphics card to use in the Mac.
PC graphics cards won't work in Macs at all, unless the ROM is designed for the Mac, or the card is flashed for use on the Mac. There's just a small number of cards where that works, and the resulting card is often unstable on the Mac. That would be another explanation for the non-support of the wide screen display on the DVI output, as the flashing software may never have been a complete changeover - perhaps leaving out some functionality that was originally provided for a Windows PC.