There are two requirements you need to meet before you can image a modern PC.
The first, which you appear to be addressing, is ensuring that the correct NIC driver is available to WinPE, but just make sure that it supports Vista, as the version of WinPE used by GSS2.5 is based on the Vista release.
The second, which may also be required, is to ensure that you have the correct driver for the SATA hard disk, in case WinPE does not natively support the SATA controller in the DELL.
Since the DELL comes with Windows 7, you may find the following utility useful to capture all the PNP device ID's:
https://www-secure.symantec.com/connect/articles/utility-assist-identifying-plug-and-play-drivers
Once you known the PNP device id for the NIC, you can quickly check if a specific driver supports this hardware by searching in the INF file for the device id.
Also, if you want to get a bit more "into" the operation of WinPE, have a look at this article:
https://www-secure.symantec.com/connect/articles/readyadventures-winpe
Just bear in mind that modern machines with SATA devices are not going to work with PC-DOS as DOS does not understand SATA and will not be able to provide access to the hard disks, or the optical drives, as these are all SATA in new machines.
Finally, when you say you purchased the latest version of GSS - are you sure you actually have the latest version? What is the build number of the version you are working with? Have a look at this thread to see what others are having to do:
https://www-secure.symantec.com/connect/forums/gss25-not-updating-live-update