DM500-S/C/T and DM500+ does not use the same images!
DM500-S/C/T uses *.img images.
DM500+ uses *.nfi images.
The best way to separate them:
DM500+ has a modem connector on the back plate, and a security chip on the moteherboard (simcard).
DM500-S/C/T does not have those two.
DM500+ has a secondstage bootloader like all the newer Dreamboxes.
Its been ages since I had a DM500+, but I think you can flash it through webif in your web browser:
http://www.sat-universe.com/showthread.php?t=139907
You just need to know the IP, or look it up in the routers webif.
Or you could do it the old way, with a nullmodem cable and DreamUp.