I thought the 3xp chip was mainly to help data encryption. Also, one of my friend's friend had a heck of a time getting it to work properly in Windows (98 or ME), and spent several minutes on the phone with Microsoft (I forgot which company's fault the problem was) with configuration and some registry changes (if I remember correctly).
I have yet to see first-hand how the 3Com card with 3xp chip performs.
If you want to compare the Intel Pro/100+ Management (which I have) to a 3Com 3c905B (which I also have), then I would recommend (not heavily) the Intel card over the 3Com. The reason - while both have similar performance with latency and tranferring files, the 3Com card took up more resources than the Intel card. Using the same 3c905B in a K6-2 300 machine, Celeron 300A@450 machine, and now a PIII-650@950 machine, there was a noticeable drop in FPS when benchmarking with Quake, Quake2, and Quake 3 (these were the only series that stayed with me through all of my comptuers). For example, if the network cards were idle, I would get ~5 FPS more (timedemo benchmark in Quake/Quake3) with the Intel than the 3Com 3c905b. If I started transferring files over the LAN here, then the difference became greater, where the result with the 3com card would drop even more.