Does the screen go black? - In this case, it could be memory (or lack thereof) in the machine. Could also be an odd video driver. You can try adding memory or adding a video driver to the WinPE image, then updating your PXE rep.
Does the MTFTP progress across the screen? If the download doesn't go, there might be a firewall or network problem interfering with MTFTP transfer.
I forgot this discussion was still out there.
I ended up turning off MTFTP service on the PXE server. Apparently Dell did not have a Bios upgrade out there to fix this yet.
I encountered this same issue on all my PXE reps for several Latitude and Optiplex models. All of my affected models have received BIOS upgrades to remedy this now (Intel boot agent 1.5.5, IIRC). I had to disable MTFTP for a few months, but I re-enabled it after the BIOS upgrades were in place.