Building boot packages in Ghost Solution Suite 2.5 is always done through the Ghost Boot Wizard, as covered in the product documentation and tutorials. Not surprisingly, this applies to the PXE boot packages as well, although the fact that the Boot Wizard hides the PXE options if 3Com boot services are not installed means that people often don't make the connection that it's the Boot Wizard that builds these.
So, you need to build a new boot package with the right driver (or one of the "Universal" drivers, if using DOS, or a multicard template, or one of the other variations) using the Boot Wizard, and then you need to ensure your PXE configuration is set up to deliver that newly-built package to the suitable machines.
A small piece of basic PXE theory of operation: PXE booting is a multistage process, just as disk booting is, with a succession of boot loaders (and menus, and what have you). The .N12 files are, by convention, the actual boot packages as specified in the PXE protocol, which means they are very limited in size and are expected to contain a real-mode 8086 secondary boot program which then uses the basic UDP and TFTP services provided by the Boot ROM to go through a more elaborate boot process of some kind.
So, the .N12 files are not (or at least, not if people follow the naming convention) the real things you ultimately boot, but rather loaders. Each .N12 loader uses a process for the next stage appropriate to the operating system being booted, and thus the layout of the TFTP server's files and directories that it expects to find and use for the next stage.
In the case of 3Com Boot Services and PC-DOS boot packages, the 3Com code contained a package builder and second-stage boot loader which the Ghost Boot Wizard uses; the GBW-built packages are in effect hard disk images with a special header attached, which the 3Com-supplied second-stage loader knows how to install as a RamDISK. The GBW thus builds a sector image of the PCDOS hard disk with the OS and drivers, and attaches the various parts licensed from 3COM to suit 3COM's environment.
In the case of Linux boot packages, a different .N12 boot loader and hard-disk image format are used, while for Windows PE yet another .N12 boot loader and hard-disk image format (in this case, .WIM files) is used. In all these cases, the Ghost Boot Wizard knows the conventions involved in building the packages, but it's up to you to install them and ensure that the correct boot loaders are used.