3Com cards are active ones (with CPU, RAM; doing protocoll handling etc.) while most other cards are passive, meaning that the CPU must do most of the work. Active cards are better especially for slower PCs.
At what processor speeds does the difference become negligible? Am I better off replacing my PII and below PCs, for example, with 3Com NICs?
You can tell when the difference became irrelevant by the decline and eventual takeover of 3Com 😜
For a 486, the difference between a basic NE2000 (i.e. RTL8009/8019) and 3Com was huge. For a P2 it would be noticeable. In later P3s it was measurable in online games, but only relevant if the CPU was really being maxed and every FLOP mattered. By the time of the AthlonXP and P4 it was only relevant if you were maxing your NIC at the same time as well (those were the days before Netflix...).
3Com was by no means the only active card vendor. But they had the best driver support, at least under old OSs, and - just as importantly - the premium you paid for all that was relatively modest: more than the low-end dumb controllers, but far less than 'enterprise grade' (eg. Intel) stuff.
Bottom line now:
- the cards are easy to find
- the drivers are easy to find
- they're easy to get running
- performance is good
Although I frequently grumble at 3Com fetishism, when it comes to 16b ISA I still have not seen a card that in is in any way superior to the 3C509B.
The same does not apply to 8b ISA - the 3C501 was famously brain-dead and the 3C503 was better only in that it actually worked. PCI 3Com cards were good, but far less compelling than at 16b ISA - also because Intel & AMD had serious alternatives for acceptable prices.