Many hours were wasted by me and others because I didn't know this about the optiplex gx270 sff computers. I went on many sites, some by Dell, trying to make them pxe boot into ubuntu 10.04 via LTSP, but while I could follow the directions easily and rechecked many times, they didn't boot often and only by going the F12 in the BIOS. So anyway, FYI, dell optiplex gx270 pxe boot does not work. This is the first Pentium 4 I've found to not work.
Here's the bios info:
phoenix rom bios plus version 1.10 A03
Dell System OptiPlex GX270 Series
Bios version a03