Not sure if this will work for USB or Cardbus NIC's, but can't see why it wouldn't, so long as your chipset is in the supported list and your bios detects the device... Highly doubt that it would do anything for WiFi, though.
I use gPXE to boot older systems with cards that are either PCI (no boot-rom) or integrated (etherboot).
You'll ONLY have to install Grub to MBR, create VERY small boot partition on HDD (50-100MB is what I use), and place gPXE image in the boot partition, direct Grub to it as if it were a standard Linux Kernel...
I use the image generator at:http://rom-o-matic.net/gpxe/gpxe-0.9.3/contrib/rom-o-matic/
I always select gpxe:all-drivers in the drop-down for a generic load, and it gives me PXE on ALL my old garbage!! Haven't run into a 10/100 NIC yet, that it wouldn't boot. I'll have to play with it in the next few days, and see what I can get it to do with wireless devices!!
BTW, in the case that this does work for ya', you've got the rest of the drive for use as swap-space... Not enough room for much else, I guess.