The reason I asked about whether you had any sort of operating system installed on these machines is that with a fully configured manufacturer's O/S installed, you could pull out the plug and play IDs for all your devices including the NIC using the code in:
https://www-secure.symantec.com/connect/articles/utility-assist-identifying-plug-and-play-drivers
Once you have the PNP ID, you can find out whether the INF file associated with any drivers you obtain, actually supports the PNP ID in your equipment. You would require the 32 bit driver for the WinPE version used by GSS, even if your operating system installation is the 64 bit version of Windows 2003 server.
If DELL are not offering a Vista driver, I would be inclined to try any available 32 bit driver from XP onwards.
Out of interest, how do you intend to image your 3000+ servers? Using a push method or using PXE boot?