I don't have a GX620 to test right now but I think I might have an answer for this.
I at one time had problems with GX620 crashing in sysprep the way that you described. I was using the "Drivers via CD" method from http://unattended.msfn.org/unattended.x … 45b91539b/ . I think driverpacks method 2 works this way, which is not compatible with the Optiplex GX620.
I found this post on MSFN that solved the issue for me "GX620 hang at first boot" http://www.msfn.org/board/GX620-hang-at … 54211.html .
The way I solved this problem was by installing the drivers via oempnpdrivers via winnt.sif. Or you could probably input the driverpath to the registry with sysprepdriver scanner.
Hope this post helps.