Good question, as I'm interested to know more about that. I have tried several NICs, such as the Intel PRO/100 Intelligent Server Adapter, the D-Link DFE-550TX and the 3Com 3C905C-TX-M, but none have provided any real offloading to the CPU or at least a tangible one. Instead, all I got was worse performance, and slow boot times caused by the interface that controls the offloading chip.
Ironically enough, the least worse of the cards I tried is the D-Link DFE 538TX, which is based on the Realtek 8139 chip. And if they performed so bad on a Tualatin, I cannot imagine on a Super Socket 7 system... But my experience might have been due to XP, instead of 98. Maybe it gets better with the VXD drivers, but with WDM ones, it's been a disaster.
My Retro Daily Driver: Pentium !!!-S 1.7GHz | 3GB PC166 ECC SDRAM | Geforce 6800 Ultra 256MB | 128GB Lite-On SSD + 500GB WD Blue SSD | ESS Allegro PCI | Windows XP Professional SP3